All patients were examined by at least one experienced clinical geneticist, who ruled out the presence of specific dysmorphic features. For all patients, the birth weights and history of postnatal growth were within the normal range, and the head circumference was normal at birth. Mental retardation was unexplained in these patients despite standard investigations, including karyotyping, subtelomeric fluorescence in situ hybridization analysis, or comparative genomic hybridization targeting regions associated with known syndromes, molecular testing for the common expansion mutation in FMR1, and computed tomography or magnetic resonance imaging of the brain.
