Genes whose primary function is not associated with epigenetic and chromatin regulatory mechanisms, such as ubiquitin-conjugating enzyme E2 A (UBE2A) and spermine synthase (SMS) in X-linked syndromic forms of mental retardation—Nascimento and Snyder–Robinson types, respectively [44]—have also shown evidence of unique episignatures. This evidence concerns the gene SMS and Intellectual disability.