FMR-1 related disorders include fragile X syndrome, FMR-1-related premature ovarian failure (POF), and fragile X-associated tremor/ataxia (FXTAS). These disorders are due to a repetitive sequence of DNA (CGG repeat) that leads to an expansion of the FMR-1 gene. A normal FMR-1 gene has 5-40 CGG repeats. 59-200 repeats are considered premutations and >200 CGG repeats is a full mutation and causes fragile X syndrome. An estimated 1 in 4000 males are affected with fragile X syndrome. Features include developmental delay, learning disabilities, attention deficit, speech delay, gross and fine motor delay, and autistic like behavior. Many females are unaffected carriers of the full expansion, however, females can exhibit symptoms as well. Female carriers of a premutation can have FMR-1 related premature ovarian failure. Male premutation carriers can develop FXTAS, characterized by white matter lesions on MRI causing intention tremor or gait ataxia.

A second gene, FMR-2, also known as Fragile X E, is a rare cause of x-linked mental retardation. Greater than 200 GCC repeats is a full mutation and causes Fragile X E syndrome.