In this study, we demonstrate that as a direct remnant of previously living cells, hair retains a vast amount of transcriptional data reflective of incomplete degradation of RNA following apoptosis. In addition to hundreds of miRNAs and snoRNAs, sequencing of hair RNA identified 7,193 unique mRNAs, or an equivalent to over a quarter of all genes in the human genome. Hair RNAs potentially reflect months of expression data temporally deposited along the length of the hair shaft and may be useful for investigations of gene expression and biomarker discovery.
As a model for studying differentiation and cell death, the hair provides insights into the extent to which RNAs are targeted for degradation during cornification. The numerous RNAs identified in the hair shaft suggest that RNA degradation is highly inefficient during cornification. In support of this hypothesis, read coverage between hair shaft versus viable keratinocyte RNA libraries exhibited remarkable similarities in coverage of intragenic regions with small biases toward reduced exonic coverage in hair shaft. Several RNases were detected in the hair shaft RNA library, including RNASE4, 7, 12, 13, RNASEN
, and RNASET2
, indicating that lack of RNases do not account for the persistence of RNAs in hair. RNA stability in hair might involve other mechanisms such as sequestration of RNases, presence of endogenous RNase inhibitors, protection of RNA by ribonucleoprotein particles or low water content in hair 
. Understanding the contribution of these potential mechanisms may be important in improving RNA detection and stability in future hair RNA expression studies. The removal of DNA versus RNA during apoptosis may play distinct roles in different tissues 
. In erythrocytes, corneocytes, and lens fiber cells, where nuclei and DNA are removed during terminal differentiation, persistence of RNA may be important in maintaining cell function. Whether incompletely removed RNA in hair shaft serves a functional purpose is unknown. It is unclear if mRNAs identified in the hair shaft are available for translation as the degree of fragmentation and survival of translational machinery were not examined in this study. In addition, studies aimed at determining whether RNA persists in other cornified tissues such as the nail could be of great importance for studying of diseases that affect localized body sites, such as cancer, infection, injury and exposures.
The discovery of stable mRNAs and miRNAs distant in the hair shaft from the scalp overcomes a formidable obstacle in the application of RNA diagnostics. Currently, RNA studies based on tissue biopsy and phlebotomy are vulnerable to RNA degradation 
. In the current study, we found that older, more distal regions of hair still contain detectable mRNAs and miRNAs. These findings have several implications. First, the stability of RNA in older portions of hair suggests that RNA may be stable in hair over many months. This attribute differs greatly from other tissues and blood and potentially resolves a problem in storage and transportation of RNA, necessary for RNA-based diagnostic or biomarker studies. Second, temporal changes in gene expression either physiologic or induced by chemical, drug or disease might be stored co-linearly along the length of the hair shaft. Use of this spatiotemporal pattern of RNA deposition might provide a novel approach to studying the natural course or inciting events of disease. In addition to these characteristics, the continuous replacement of hair and its ease of access are advantages to developing diagnostic approaches based on hair RNA.
While possibly ideal for many types of biomedical studies, the use of hair RNA for molecular studies and diagnosis currently has several limitations. At this time, the amount of variation in RNA expression between different individuals, ages, and genetic backgrounds are not known. In addition, differential growth rates of hair in individuals due to differences in genetic background, age, and other factors obscure accurate measurements of time. Recent studies indicate that growth of human hair may vary from 1.3 to 2.2 months per centimeter in different individuals 
. More accurate measurements might be made possible with the identification of cyclically expressed genes, which could used to normalize differential growth rates 
. An additional limitation is that since new cells are added to the hair shaft only during active hair growth, it is not yet known what the effects different stages of the hair cycle might have on the pattern and stability of RNA in the hair. In this case, identification of transcripts representative of the final stages of the hair cycle might be required to determine whether retained transcripts reflect specific portions of the hair cycle. Lastly, because the characteristics that contribute to RNA stability in the hair are unknown, it is possible that RNA stability varies in individuals. These unknown aspects may bias the results of RNA detection in the hair.
The use of parallel sequencing of small RNAs provides an atlas of residual transcripts in humans of different genders, populations, and hair type. Sequencing technology provides several informative features valuable to molecular diagnostic studies including transcriptional orientation, multiple means of validation of expression and quantity of transcripts, and sequence data 
. Sequence data also provide a significant source for the identification of genetic polymorphisms, detection of allele-specific expression differences, and somatic mutations. The extent to which variant alleles are present in hair shaft RNA was not explored in this study. Current obstacles to use of this data for allele-specific studies include the possibility of chemical changes in nucleotides, e.g. cytosine deamination 
, and incorporation errors inherent to RNA polymerases 
. Further studies are needed to assess the impact of these factors on the fidelity of hair RNA sequences.
In the past, hair analysis has aided in the diagnosis of inherited and acquired diseases 
and as a phenotypic marker of medication and chemotherapeutic response 
. Thus while the discovery of post-apoptotic RNA stability in hair provides insights into programmed cell death, these findings also greatly expand the possible applications of hair in medical discoveries in the clinic and the lab. The unique properties of the linear growth of the hair serve as a record of gene activity during organ growth and of the individual's own history. This record may be of great value for studies in organ development, evolution, genetic variation, and population genomics.