We describe two cases of prenatally ascertained supernumerary marker chromosomes derived from chromosome 6 [SMC(6)]. Prenatally ascertained SMCs derived from chromosome 6 are rare with only nine cases reported. Seven of these cases are summarized in . We have excluded from the prenatal case reported by Chen et al. 
in which the proband had multiple SMCs including an SMC(6), since the phenotype is likely confused by the presence of other abnormalities. Of interest, Liehr et al. [2006b]
noted that SMC derived from chromosome 6 are over-represented in cases with multiple SMCs (33%), although the significance of this observation is unknown. The prenatal SMC(6) case of Sala et al. 
was also excluded as it was a neocentrome-containing SMC derived from distal 6q, 6q26→qter, whereas the majority of SMC(6) contain variable amounts of pericentromeric chromosome 6 material (). We have included the case reported by Crolla et al. 
(case 6 in ) as it reports prenatal findings despite a postnatal ascertainment.
Reported cases of prenatally diagnosed supernumerary marker chromosomes derived from chromosome 6.
Comparison of the prenatally diagnosed cases of SMC(6) to date shows some karyotype-phenotype correlation (). Patients fall into three broad categories; no detectable euchromatin with a normal phenotype, detectable euchromatin with a normal phenotype and detectable euchromatin with an abnormal phenotype. Where the SMC(6) were characterized for euchromatic content, the genotype-phenotype comparisons summarized in are consistent with observations from a larger series of postnatal SMC(6) cases reported by Liehr et al. [2006a]
; the absence of euchromatin or only small amounts of euchromatin from the centromere to distal 6p12.1 or proximal 6q12 is associated with a normal phenotype, whereas abnormal phenotypes and/or development delay are associated with euchromatin content extending beyond these regions Liehr et al. http://www.med.uni-jena.de/fish/sSMC/06.htm#Start06
]. Three cases in (cases 3–5) fall into the normal phenotype categories with no or minimum euchromatin within the region of distal 6p12.1 to proximal 6q12. Two of these reports [Bartsch et al., 2005
; Liehr et al., 2006a
] described small SMC(6) with no detectable euchromatic content with FISH. Both pregnancies went to term and resulted in normal infants. Phenotype and development were normal in both patients at the time of follow up. These cases are consistent with the observation that minute SMCs with no detectable euchromatin have a low risk of phenotypic abnormality [Cotter et al., 2005
]. The 3rd
patient reported by Leite et al., 
with the SMC(6) containing 6cen→6p12 had a normal phenotype and is also consistent with the genotype-phenotype predictions mentioned above [Liehr et al, 2006a
Concomitantly, the presence of a large amount of euchromatin extending beyond distal 6p12.1 and proximal 6q12 is associated with an abnormal phenotype. One of the previously reported cases (, case 6) and case 1 of the current study fall into this category. The SMCs in Case 1 of the current study and the case reported by Crolla 
contained the proximal short arm region of chromosome 6 and had abnormal phenotypes. Therefore, trisomy for the short arm of chromosome 6 for at least 25 Mb (6cen to CTD-2118F18) appears to be associated with an abnormal phenotype. One of these cases [Crolla, 1998
] had some of the features associated with uniparental disomy of the normal chromosome 6 homologs. This SMC(6) was maternal in origin and UPD studies showed the patient to also have paternal UPD6. The patient did have transient neonatal diabetes which is associated with patUPD6. However, the presence of extra euchromatin in the marker chromosome most likely contributes to the other abnormal manifestations.
The remaining two cases of SMC(6) with uncharacterized euchromatic content presented with normal phenotypes (, cases 2 and 7). Case 2, presented here, had a normal phenotype, possibly due to the lower level of mosaicism of the SMC(6), or perhaps less euchromatin present that was not detected by the array. Array CGH has been shown to identify mosaicism down to 20% in some cases [Ballif et al., 2006
]. In this case, cultured lymphocytes showed 40% of metaphases with the SMC. However, whether this percentage is representative of all nucleated cells from which DNA was extracted is unknown. The case described by Hastings et al. 
was clinically and developmentally normal at 5 months. The SMC(6) in this fetus was also present in the phenotypically and developmentally normal mother.
Secondary to euchromatin characterization of SMC(6), which is an important predictor of phenotype, there are other variables that might affect the severity of the phenotype in SMC(6) patients. UPD testing may be warranted as a SMC(6) may be the result of a trisomy rescue event secondary to a chromosome 6 non-disjunction. PatUPD6 is associated with transient neonatal diabetes [James et al., 1995
], as in the patient reported by Crolla et al., 
. Postnatally, UPD6 testing is likely only warranted when transient neonatal diabetes is present. However, UPD6 analysis should be considered in all prenatal SMC(6) cases. Parental karyotype analysis should also be performed to determine whether a SMC is de novo
or familial, as the presence of the same SMC in a phenotypically and developmentally normal parent makes a normal phenotype in the fetus more likely. However, care should be taken with this approach, as the tissue distribution and percentage of the SMC in the fetus may be different from that in the parent. The tissue distribution of the SMC can also affect the severity of the phenotype. A low or high percentage of cells with a SMC in one tissue may not necessarily correlate with a low or high risk, respectively, of an abnormal phenotype. It appears that the most accurate predictor of phenotype is the euchromatic content of a SMC(6). Currently, chromosome microarray analysis is the most effective approach to characterize the euchromatic content of a SMC. However, if the percentage of cells with the SMCs is below the threshold of detection of array analysis, as may be the situation for Case 2 presented here, it may be uninformative. Higher density arrays have enriched coverage over the pericentromeric regions, and may have identified euchromatin on the SMC of Case 2. Some commentators have raised concerns over the prenatal use of high-density arrays citing the higher incidence of variants of unknown significance (VOUS) which raise uncertainty of phenotypic outcome [Lichtenbelt et al., 2011
]. The use of more targeted arrays with lower incidence of VOUS are recommended in prenatal diagnosis [ACOG Committee on Genetics, 2009
]. Alternative strategies, such as the acro-FISH approach and the use of pericentromeric FISH probes, may be more effective for characterization of SMCs with low level mosaicism [Liehr et al, 2006a
; Starke et al., 2003
]. Additionally, enrichment of SMC DNA by microdissection for array CGH has also been reported (Backx et al., 2007
In conclusion, array CGH is a powerful tool for the characterization of SMC. Together with the use of targeted FISH approaches in cases of low level mosaicism for SMC, it has led to a more accurate genotype-phenotype correlation. For future cases, these tools should be used in all cases of SMC, particularly prenatally ascertained SMC. The presence of mosaicism in a large number of these cases further adds to the complexity in predicting the outcomes. Therefore, the continued investigation of a large number of SMC cases and a better understanding of the genetic content of the SMC is important for improved delineation of karyotype-phenotype correlation.