Fragile X syndrome, a common form of X-linked intellectual disability (ID), is associated with expansion of a CGG trinucleotide in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene located on the X chromosome at band q27.3. Unaffected individuals have between 6 and 54 CGG repeats. Individuals affected with ID have > 200 CGG repeats. It has become apparent that 2 clinically distinct phenotypes are common among carriers of alleles with 55 to 200 CGG repeats (called premutations). Female carriers are at risk for premature ovarian failure, while both male and female carriers appear to be at risk for a late-onset neurological disorder called fragile X-associated tremor/ataxia syndrome (FXTAS), with males being more frequently and more severely affected than females. Penetrance of FXTAS-associated variants is incomplete. The commercially available tests for FXTAS are identical to those for fragile X syndrome, differing only with the clinical criteria for referral.