polymorphism in SMN1 , g.27134T > G, NG_008691.1, reported to be a haplotype-specific marker for (2 + 0) carriers. We retrospectively reviewed test results from the carrier screening and diagnostic ddPCR assay ran from March 2018 to August 2019. In total, we collected SMN1 and SMN2 copy number data for 4,438 individuals. We then performed g.27134T > G ddPCR analysis on samples that tested positive for 5 or more copy numbers of SMN1. We determined that 5 copy numbers of SMN1 occurs with a frequency of 0.05%. g.27134T > G ddPCR analysis of these 2 individuals resulted as 3/5 and 1/5 copy numbers positive for the g.27134T > G polymorphism. Our findings have important implications in carrier screening. Our data suggests that the status of the individuals with 5 copy numbers in our study likely include a (3 + 2) status as they both tested positive for the polymorphism. This reveals the possibility of silent SMN1 (3 + 0) carriers amongst individuals testing positive for 3 copy numbers of SMN1 in carrier screening. Furthermore, atypical copy numbers of SMN1 can affect the risk and clinical severity of other neurological disorders including amyotrophic lateral sclerosis and progressive muscular atrophy. This highlights the importance of determining the prevalence of high SMN1 copy number status in the general population.
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