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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Cancer Epidemiol Biomarkers Prev. Author manuscript; available in PMC 2009 September 1.
Published in final edited form as:
PMCID: PMC2559953

Sexually Transmissible Infections and Prostate Cancer Risk



Sexually transmissible infections (STIs) have been variably associated with increased risks of prostate cancer, largely in case-control studies.


In the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, we examined risk of prostate cancer in relation to serum antibodies to Chlamydia trachomatis, human papillomavirus (HPV) types 16 and 18, herpes simplex virus (HSV) type 2, cytomegalovirus (CMV), and human herpesvirus 8 (HHV-8) in 868 cases (765 whites and 103 blacks) and 1,283 controls matched by race, age, time since initial screening, and year of blood draw; all blood samples were collected at least one year prior to prostate cancer diagnosis, except for 43 black cases. We also assessed risk associated with self-reported history of syphilis and gonorrhea.


Prevalences of the 7 STIs among controls were weakly correlated, and all were more frequent among blacks than whites, except for HHV-8. Among whites, prostate cancer risk was not significantly associated with the individual infections nor with their number (Ptrend = 0.1); however, men with one or more STI had slightly higher risk (odds ratio [OR] = 1.3, 95% confidence interval [CI] = 1.0-1.6). Among blacks, excess risk was associated with IgA antibody to C. trachomatis (OR = 2.1, 95% CI = 1.2-3.6).


This large prospective study of prostate cancer shows no consistent association with specific STIs and a borderline association with any vs. none. Whether a shared response or correlated infection not directly measured underlies the weak association requires further study.


Prostate cancer is the most common cancer among American men, but its risk factors, aside from older age, African American race, and family history, are poorly understood (1). Causes for the racial disparity in prostate cancer incidence are unknown. Cultural variations in environmental and lifestyle factors, including male sexual behavior have been suspected to play a role (2;3).

In a meta-analysis of mostly case-control studies by Dennis et al (4), self-reported history of sexually transmissible infections (STIs), particularly syphilis and gonorrhea, was associated with increased prostate cancer risk (OR = 1.4, 95% CI = 1.2-1.7), as were an increased number of sexual partners and higher frequency of sexual activity (4). Combining self-reported and laboratory data, a subsequent meta-analysis by Taylor et al (5) found elevated prostate cancer risks associated with gonorrhea, human papillomavirus (HPV), and any STIs (ORs ranging from 1.4 to 1.5, all statistically significant). However, subsequent large prospective studies (with 691 (6;7), 738 (8), 804 (9), and 2,263 (10) prostate cancer cases) found no association for self-reported history of syphilis and gonorrhea (10), no association for seropositivity of HPV-16, HPV-18, and HPV-33 (7;9), no association (7) or a reduced risk for Chlamydia trachomatis seropositivity (OR = 0.7) (8), and a reduced risk for human herpesvirus 8 (HHV-8) seropositivity (OR = 0.7) (7), although one found an increased risk for Trichomonas vaginalis seropositivity (OR = 1.4) (6). In tissue-based studies, the presence of HPV (summarized by Taylor et al (5)), C. trachomatis (11;12), HHV-8 (13), herpes simplex virus (HSV) type 2 (14;15), and cytomegalovirus (CMV) (14;16) has been reported in prostate cancer or other prostate samples. However, other studies have failed to detect these agents (5;17-20), potentially reflecting tumor sampling artifacts, DNA degradation, PCR contamination, or even a “hit and run” effect (21).

These prior studies focused on one or a few STIs at a time with limited ability to address joint effects. Furthermore, more data from large prospective investigations could help confirm or refute the suggestive case-control associations. We therefore examined prostate cancer risk associated with serum antibodies against C. trachomatis, HPV types 16 and 18, HSV type 2, CMV, and HHV-8, and with self-reported history of syphilis and gonorrhea, using prospectively collected serum specimens and questionnaire data from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial.

Materials and Methods

The PLCO Trial

The PLCO Cancer Screening Trial is a randomized community-based study designed to evaluate the effectiveness of cancer screening tests on site-specific cancer mortality. Detailed descriptions of the Trial were provided elsewhere (22-24). Briefly, about 77,000 men and 77,000 women aged 55-74 years were recruited to the Trial through direct mailings, advertisements, and other means. Enrollment took place between September 1993 and July 2001 at ten screening centers (Washington, DC; Detroit, MI; Salt Lake City, UT; Denver, CO; Honolulu, HI; Minneapolis, MN; Marshfield, WI; Pittsburgh, PA; St Louis, MO; and Birmingham, AL). Non-Hispanic whites and blacks comprised 88% and 5% of the participants, respectively. Approximately 50% of the participants were randomized to the intervention arm, which for males included periodic screening tests for prostate cancer (PSA annually in years 0-5 and digital rectal exam (DRE) annually in years 0-3). Data on general risk factors and dietary intake were collected through questionnaires, and serial blood samples were obtained for etiologic and early marker studies (25;26). Institutional review boards at each of the participating institutions approved the PLCO protocol and participants provided written informed consent.

Case Ascertainment

Prostate cancer cases in the PLCO Trial were identified through multiple mechanisms and confirmed by pathological records. A positive screening test for prostate cancer (PSA > 4 ng/mL or a DRE suspicious for cancer) led to referral for clinical work-up which may include a biopsy. In addition, participants reported annually by mailed-questionnaires whether they had a diagnosis of cancer in the previous year and the type and date of the diagnosis. Reports from physicians and relatives were also collected when available. The National Death Index (NDI) and cancer registries as available were periodically searched to enhance completeness of endpoint ascertainment. For subjects with suspected or reported prostate cancer, medical records, including clinical and pathologic data pertinent to the date of diagnosis, stage and grade (Gleason score), etc. were obtained and coded by trained abstractors.

Nested Case-Control Study

Subjects were eligible if they had been randomized to the intervention arm, were non-Hispanic white or black men, had a valid PLCO screen for prostate cancer (PSA or DRE) between October 1993 and September 2001, completed the baseline risk factor questionnaire, provided a blood sample, signed the informed consent, and reported no prior history of prostate cancer (n=28,243) (27-36). Case subjects were men with a pathologically-confirmed diagnosis of prostate adenocarcinoma. Among whites, we included only cases diagnosed one year or longer after their first valid prostate cancer screen (n = 765). Among blacks, we had 60 cases diagnosed at least one year after the screening; to increase power, we also included 43 cases diagnosed at enrollment or within one year of screening for a total of 103 black cases. Cases diagnosed with stage III-IV or Gleason score of 7 or greater were considered aggressive. For comparison, 1,283 men without a diagnosis of prostate cancer at the time of case diagnosis were selected as controls using incidence density sampling, frequency-matched by age (55-59, 60-64, 65-69, 70-74), ethnicity (1.2:1 for whites, 4:1 for blacks), time since initial screening (one year interval), and year of blood draw. All subjects were followed from their first valid prostate cancer screen to first occurrence of prostate cancer, loss-to-follow-up, death, or September 30th, 2001, whichever came first. Both cases and controls had an average follow-up of 1.6 years with means of 1.4 post-enrollment screening visits prior to diagnosis/selection.

Assessment of Syphilis and Gonorrhea

At study entry, participants completed a risk factor questionnaire reporting whether a doctor had ever told them that they had syphilis or gonorrhea. We compared these cases both to controls in the nested case-control study and to all at-risk PLCO participants in the intervention arm. Other information collected in the baseline questionnaire included marital status, education, weight, height, use of non-steroid anti-inflammatory drugs (NSAIDs), smoking behavior, family history of prostate cancer, and history of enlarged prostate or benign prostatic hypertrophy, prostate biopsy, and vasectomy.

Serological Assessment

Serum samples were collected from intervention arm participants at study entry, processed within two hours of the blood draw, and stored at -70°C or lower. Aliquots of 100 ul serum were sent to five separate laboratories for assays, standardized with predetermined cutoff points, categorizing the infection status as positive, negative, or equivocal. Laboratory personnel were blinded to case-control status. Thirty-three masked replicate samples from 4 individuals were tested to assure satisfactory performance for each serological assay before proceeding with the study samples. All assays, except HHV-8, were performed in duplicates, and a third test was performed for discordant calls; for each batch, internal positive and negative quality control samples were tested to ensure quality performance of the assays. For HHV-8, three positive and three negative controls were run on each plate comprised of 96 samples; the control values were required to be within a narrow range or the plate was repeated. We also interspersed 96 pairs of blinded duplicate samples (from 48 whites and 48 blacks) among the study samples to evaluate assay reproducibility. For 50 of the controls, we added a second sample collected a year after the baseline serum, placed immediately adjacent to a subject's corresponding baseline sample to test intra-individual variability.

Chlamydia trachomatis

Using a commercial enzyme-linked immunosorbent assay (ELISA; Medac, Germany), we measured immunoglobulin A (IgA) and immunoglobulin G (IgG) antibodies to a synthetic peptide derived from the major outer membrane protein (MOMP) of C. trachomatis that is common among all serotypes; sensitivity and specificity of these assays have been reported to range from 79-88% and 82-89%, respectively (8;37). Also, we tested subjects seropositive for IgA or IgG antibodies to MOMP for IgG to C. trachomatis heat shock protein 60 (HSP 60) as a presumptive indicator of repeated or persistent infection (38). An OD cutoff value of greater than 1.1 indicated IgG and IgA antibodies to C. trachomatis and IgG antibodies to HSP60. HPV types 16 and 18: We used a virus-like particle (VLP)-based enzyme immunoassay to measure serum IgG against HPV types 16 and 18 capsids, as previously described (39). The VLPs were produced in the Trichoplusia ni (High Five) cells (Invitrogen, Carlsbad, CA) from a recombinant baculovirus that expresses the L1 gene of HPV-16 or HPV-18, and purified by density-gradient ultracentrifugation as described previously (40). The cutoff for seropositivity was defined as an OD value greater than the mean plus/minus 3 standard deviations of serum samples from children, ages 1 and 5 years, after exclusion of outliers. HSV-2: HSV-2 IgG antibodies were measured using a solid-phase enzymatic immunodot assay with a purified glycoprotein specific for HSV-2 (gG2) as the antigen (41;42). The assay has been tested widely for a large number of samples collected in the NHANES II and NHANES III with demonstrated sensitivity over 98% and specificity over 99% (41). CMV: CMV IgG antibodies were measured using commercially available Microparticle Enzyme Imunoassay (MEIA, the AxSYM CMV IgG assay, Abbott Laboratories) and the manufacturer-recommended cutoff for seropositivity. The presence of at least 15 Antibody Units per mL (AU/mL) of sample is indicative of past or current infection with CMV. Results of equal or greater than 10 AU/mL but less than 15 AU/mL are considered equivocal. The relative sensitivity was 99.7% and specificity was 100% according to the manufacturer (43;44). HHV-8: Antibodies to HHV-8 were measured by ELISA to detect IgG against the HHV-8 K8.1 structural glycoprotein expressed during lytic viral replication as previously described (45;46). The cutoff for seropositivity was calculated as the mean of the three negative controls on the plate plus 0.75, in order to correct for plate to plate variability.

Statistical Analysis

We calculated modified kappa coefficients for agreement between the blinded duplicates as suggested (47). To evaluate the correlation between the STIs, we calculated Spearman correlation coefficients and P values from chi-square tests. Conditional logistic regression, conditioning on the four matching factors of age, ethnicity, time since initial screening, and year of blood draw, was used to estimate ORs and 95% CIs for the association with each STIs, including analysis on any STI. Additional adjustment of potential confounding factors, including family history of prostate cancer, education, NSAIDs use, smoking, and marital status, yielded virtually no change in results, thus results were not presented herein. We also performed unconditional logistic regression models and the results were very similar (data not shown). The small numbers of serologic tests classified as equivocal were analyzed as positives; in sensitivity analyses, exclusion of these samples yielded very similar results (data not shown). We constructed a summary variable for total number of past infections (ranging from 0-7) from the five serologically identified STIs (i.e., C. trachomatis by IgA or IgG, HPV-16 or -18, HSV-2, CMV, and HHV-8) and two self-reported histories (i.e., syphilis and gonorrhea). We used this variable to perform trend tests for the associations with prostate cancer risk in logistic regression models. To explore potential risk modifications by suspected prostate cancer risk factors or inflammation-related factors, we conducted stratified analyses using questionnaire data (e.g. education, marital status, family history of prostate cancer, history of hepatitis infection, use of NSAIDs such as aspirin- or ibuprofen-containing products), clinical information (e.g. stage, Gleason score, age at diagnosis), and genotypes (e.g. single nucleotide polymorphisms in RNASEL, NOS2A, NOS3, SOD1, SOD2, SOD3, IL1B, IL6, IL8, IL10, TNF, PTGS2, PPARD, and PPARG genes (28-30;32;34)). To analyze associations with self-reported history of syphilis and gonorrhea among all at-risk PLCO participants in the intervention arm, we used Cox proportional hazards regression to estimate relative risks (RRs) and 95% confidence intervals (95% CIs), adjusting for age, center, education, smoking status, NSAIDs use, and family history of prostate cancer. Two-sided p-values < 0.05 were considered statistically significant.


Table 1 summarizes the quality control data for each serologic test. For both reproducibility of blinded duplicate samples (n = 96 pairs) and intra-individual agreement of samples collected one year apart (n = 50 pairs), kappa values ≥0.7 were obtained for all assays, except the reproducibility of HPV-18 (kappa = 0.6).

Table 1
Performance evaluation of serological assays used in the study

Prevalences of the 7 STIs were weakly correlated among controls, similarly for whites and blacks (Table 2). C. trachomatis IgA and IgG antibody positivities were significantly correlated and both were positively correlated with HSV-2 seropositivity; correlation with self-reported history of syphilis and gonorrhea was slightly stronger for C. trachomatis IgA than IgG. Likewise, seropositivity for HPV-16 and HPV-18 were significantly correlated and both were positively correlated with HSV-2 seropositivity; correlation with self-reported history of gonorrhea was slightly stronger for HPV-16 than HPV-18. Seroprevalences of CMV and HHV-8 appeared to be less correlated with those of other infections.

Table 2
Correlation coefficients for prior sexually transmissible infections among nested controls in the PLCO Cancer Screening Trial, 1993-2001

Cases and controls of both racial groups were similar with respect to body mass index, smoking behavior, use of nonsteroidal anti-inflammatory drugs (i.e., aspirin or ibuprofen), history of prostatitis, and prior vasectomy (Table 3). For both racial groups, cases were significantly more likely than controls to have a history of benign prostatic hypertrophy or prostate biopsy, and had higher PSA levels and prevalence of prostate enlargement detected at study entry. Among whites, a higher percentage of cases than controls reported a family history of prostate cancer in fathers or brothers, and cases were slightly more frequent than controls to report a college degree or higher education; these characteristics did not differ significantly among blacks.

Table 3
Self-reported baseline and clinical characteristics of prostate cancer cases and nested controls among whites and blacks in the PLCO Cancer Screening Trial, 1993 - 2001

Among controls, blacks had higher seroprevalences of all studied STIs except HHV-8 (Table 4). Among whites, none of the infections individually was significantly associated with prostate cancer risk. Among blacks, IgA, but not IgG, seropositivity to C. trachomatis was associated with increased risk of prostate cancer; this association was slightly stronger (OR = 2.3, 95% CI = 1.2-4.1) after exclusion of men with weak-positive results. Among subjects seropositive for IgA or IgG antibodies to C. trachomatis, approximately 40% of whites and 74% of blacks also had antibodies to C. trachomatis HSP 60, but these percentages did not differ between cases and controls.

Table 4
Prior sexually transmissible infections and risk of prostate cancer among whites and blacks in the nested case-control study, PLCO Cancer Screening Trial, 1993 - 2001

Among whites, slightly more cases than controls were positive (seroprevalence or questionnaire-based) for at least one STI; positivity for any STI was associated with a 1.3-fold increased risk of prostate cancer (95% CI = 1.0-1.6, P = 0.05) (Table 4). The dose-response trend for number of STIs was suggestive albeit nonsignificant. Among blacks, nearly all subjects were positive for at least one STI; compared to men with none or one STI as the reference group, those with ≥2 prior STIs had a nonsignificant elevation in prostate cancer risk. Although limited by small numbers, we observed no obvious differential in risk patterns between black cases who were diagnosed at least one year versus within a year after the serum collection.

Further exploration in white men showed no differences in risk patterns for sub-groups defined by age at diagnosis, education, marital status, history of hepatitis infection, use of NSAIDs such as aspirin-or ibuprofen-containing products, genotype of selected variants in inflammation genes (PInteraction <0.05 for only 2 of the 34 variants examined), or aggressive (stage III-IV or Gleason score ≥7) vs. non-aggressive cancer. Prior STI was associated with prostate cancer among subjects without a family history of prostate cancer (OR = 1.4, 95% CI = 1.1-1.8), but not among those with a family history (OR = 0.8, 95% CI = 0.3-2.3) (PInteraction = 0.04). Sample size for black men was too small for stratified analysis.

In addition, compared to all at risk PLCO participants in the intervention arm, prostate cancer cases had no excess of either syphilis (whites: RR = 0.4, 95% CI = 0.1-1.3; blacks: RR = 0.7, 95% CI = 0.3-1.8) or gonorrhea (whites: RR = 1.0, 95% CI = 0.8-1.3; blacks: RR = 0.9, 95% CI = 0.6-1.4), similar to the comparison to the nested controls (Table 4).


Circulating antibodies against infectious agents and self-reported history of syphilis and gonorrhea represent an individual's cumulative lifetime exposure or past infections and thus are particularly suited for risk evaluation of cancer with a long latent disease process. Our analysis is among the first to examine the effects of many STIs simultaneously in a large prospective study of prostate cancer risk. We found no strong association between prior evidence of specific STIs and prostate cancer, but having had any STI was associated with a modestly increased risk among whites. In the smaller number of black men studied, we found an increased risk associated with IgA antibody to C. trachomatis, and a non-significant elevation with 2 or more prior STIs.

Race-specific seroprevalence estimates for some STIs were available from nationally representative populations, although further specification by gender and age were often not provided. Seroprevalences for whites in this study were similar to NHANES for CMV (78.2%) (48), HPV-16 (8.0%) (49) and HSV-2 (13.7%) (50), while the black controls had higher seroprevalences for the latter two agents (90.2% (48), 9.6% (49), and 40.3% (50), respectively, in the NHANES). Seroprevalence reports for HHV-8 were highly variable (51); our measure of HHV-8 K8.1 was higher for both blacks and whites than our laboratory had previously reported from NHANES (2.1% and 1.5%, respectively) (52), although it was similar to their alternative NHANES estimate (7.1%, both races) based on a less-specific cutoff (52), as used in this study. For C. trachomatis IgG and HPV-18, our results were similar to findings from large population-based case-control studies of whites (11% (8) and 3.8% (9), respectively); national survey data were unavailable for black men.

Compared with other STIs, characterization of syphilis and gonorrhea is relatively achievable through self-reported survey because these conditions are typically symptomatic in men and routinely investigated in clinical settings. The frequencies of self- reported history of syphilis and gonorrhea among our control subjects were similar to self-reports in two large U.S. studies (0.1% (53) and 0.2% (10) in whites; 2% in blacks (53)) and the seroprevalence of syphilis (0.6%, mixed races) in the 2001-2002 National Health and Nutrition Examination Survey (NHANES) (

While both serologic measures and questionnaire-based self-reports may have error, such errors should be non-differential between cases and controls, as materials and data were prospectively collected and laboratory personnel were blinded to case-control status. To the extent that nondifferential misclassification occurred, our results may be biased toward the null.

Meta-analyses (4;5) showed overall modest associations of prostate cancer with self-reported syphilis (OR = 1.4-2.3), gonorrhea (OR = 1.3-1.4), and history of any STIs (OR = 1.4-1.5). These analyses were based predominantly on case-control data for Caucasians; studies of African Americans, serological measures, and prospective data have been lacking. A large population-based case-control study of prostate cancer in U.S. blacks (479 cases) and whites (502 cases) found a 1.6-fold risk associated with self-reported history of gonorrhea or syphilis and, in a subgroup with available serum (125 black cases, 146 white cases), a 1.8-fold risk association with antibodies to syphilis (53); patterns of risk were similar for whites and blacks (53). The prospective Health Professionals Follow-up Study (10), not included in either of these meta-analyses, investigated white men with a low prevalence of self-reported history of syphilis and gonorrhea and found no association of either with prostate cancer risk.

C. trachomatis is the most common bacterial STI (54) and may cause chronic persistent infections. In sexually active men, it is the main cause of nongonococcal urethritis and has been suspected as a probable cause of prostatitis (12). C. trachomatis IgG antibodies were not associated with prostate cancer in our study nor in another prospective study in US whites (7), and an inverse association was reported in a Scandanavian study (8). However, our findings of a significant association of C. trachomatis IgA seropositivity with prostate cancer risk among blacks is suggestive. Further investigation is warranted to illuminate whether the observed racial disparity was due to differences in seroprevalence, susceptibility to infection or cancer, other differences between the racial groups, or occurred by chance.

HPV and the herpesviruses HSV-2, CMV, and HHV-8 are all sexually transmissible, although non-sexual routes have also been established for CMV and suggested for HHV-8 (52;55-58). Sero-epidemiologic associations with prostate cancer have been mixed. A meta-analysis of 8 retrospective and 2 prospective studies found a significant 1.4-fold increased risk of prostate cancer with HPV seropositivity (5), but two additional reports not included in the meta-analysis found no association with antibodies to HPV types 16, 18 or 33 (7;9). HSV-2 seropositivity was not associated with prostate cancer in a sub-sample of the Nordic nested case-control study (165 cases) (59). Using small convenience samples, findings have been mixed for associations of CMV infection and prostate cancer risk (16;60;61). Findings also were mixed for HHV-8 seropositivity (46;62), with the largest reported study (691 cases) showing a modest inverse association with antibodies to HHV-8 lytic antigens (7).

Other infections also were linked with prostate cancer risk. Seropositivity for Trichomonas vaginalis, a relatively prevalent but under-recognized STI, increased prostate cancer risk in the Health Professionals Follow-up Study (6). A novel retrovirus named XMRV was discovered in prostate cancer tissues from men carrying genetic variants of RNASEL (63). Four or more years use of tetracycline, an antibiotic used to treat severe acne, was found to be associated with increased prostate cancer risk (64).

A potential explanation for these disparate associations is that the host response of infection, specifically inflammation, represents a final common pathway for prostate cancer risk. Prostate cancer tissues frequently contain activated inflammatory cells, including the proliferative inflammatory atrophy lesions thought to be precursors of early cancer development, and may have somatic alterations in inflammation-related markers (65;66). Genetic and circulating markers of inflammation and host response to infection have been variably shown to increase prostate cancer risk, whereas intake of NSAIDs and antioxidants has been protective (67-69). These considerations implicate chronic inflammation as a potential mechanism of prostate carcinogenesis. Alternatively, the diffuse associations with the various infections may be due to correlation with a true causal factor not directly measured.

Our study has several notable features that overcome important limitations of previous investigations. Cases and controls were identified from the same source population which was screened for prostate cancer by a standardized procedure. Data and specimens were collected prospectively prior to diagnosis, and were used to assess a large number of STIs simultaneously. However, prevalences of the individual infections were relatively low in our white participants and we had few black participants which limited our statistical power. We attempted to address this deficiency by including black cases diagnosed within one year of blood draw. Potentially these cases may be more slowly progressing or less clinically aggressive than cases arising at least one year after the initial screen; nevertheless, risk patterns did not grossly differed between these two groups of black cases. White participants with at least one past infection had modestly increased prostate cancer risk; this pattern was not replicated among our black participants, but very few had no past infections.

In summary, this large prospective study found no consistent association between history of individual STIs and prostate cancer risk. Our novel finding of an increased risk with C. trachomatis IgA antibodies was restricted to blacks and requires confirmation. By studying many STIs simultaneously, we found a modest risk associated with any prior STI among whites. The modest associations and lack of specificity parallel the variable findings from previous reports and may suggest involvement of a correlated factor or shared underlying response (e.g. inflammation) not directly measured. Further studies in high risk populations would help advance our understanding of the role of STIs in the etiology of prostate cancer.


This research was supported by the Intramural Research Program of the Division of Cancer Epidemiology and Genetics and contracts from the Division of Cancer Prevention, National Cancer Institute, National Institutes of Health, Department of Health and Human Services. The authors thank Drs. Christine Berg and Philip Prorok, Division of Cancer Prevention, National Cancer Institute, the Screening Center investigators and staff of the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, Mr. Tom Riley and staff, Information Management Services, Inc., Ms. Barbara O'Brien and staff, Westat, Inc., and the staff of the serological testing laboratories. Most importantly, we acknowledge the study participants for their contributions to making this study possible.


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