FMR1 and fragile X syndrome: Fragile X syndrome (FXS) is the most common inherited form of intellectual disability (ID) affecting ~1/4000 males and ~1/5000 to 1/8000 females (Refs 1, 2) and is caused by the full-mutation (FM, >200 repeats) hyperexpansion of (CGG)n triplet-repeat sequence in the 5′-untranslated region of the X-linked Fragile X Mental Retardation 1 (FMR1) gene (Refs 3, 4, 5, 6).