Mental Retardation, Autosomal Dominant 5 (MRD5) (OMIM #612621) is caused by mutations in SYNGAP1. MRD5 is characterized by moderate-to-severe intellectual disability with delayed psychomotor development apparent in the first years of life (Holder et al., 2019). Here, SYNGAP1 is linked to autosomal dominant non-syndromic intellectual disability.