There have been major advances in the past few years in our understanding ofthe X-linked learning disabilities. The most common of these is thefragile-X syndrome, but the number of other gene defects that are nowrecognised to be linked with learning disability is increasing year on year.We describe one family displaying a rare X-linked abnormality. Repeatgenetic testing was requested for a family member with mild learningdisability when, following chromosomal analysis for her brother, it becameknown that he had a genetic defect. The genetic defect 46,Xdup(X) (p22.13p22.31) was identified. To our knowledge this is the first time this preciseconfiguration has been demonstrated. We conclude that genetic testing forindividuals with learning disability is worthwhile, even when there may beonly a low index of suspicion.