For example, a methylation-specific PCR (MS-PCR) assay for unmethylated paternal alleles used in the diagnosis of Prader-Willi syndrome has been reported to change from hypermethylation in fresh samples to gaining some unmethylated copy of SNRPN gene in samples cryopreserved for only two months at − 80°C [7]. This evidence concerns the gene SNRPN and Prader-Willi syndrome.