FMR1 and fragile X syndrome: CGG-triplet repeat hyperexpansions in the 5′ untranslated region of the X-linked fragile X mental retardation 1 (FMR1) gene cause fragile X syndrome (OMIM #300624) via FMR1 promoter hypermethylation and sequential loss of the encoded FMR1 protein (Refs 1–3).