• CMA in all individuals with ID/GDD and/or ASD • FMR1 repeat analysis in males and females with ID or a family history of ID • Depending on history and physical examination, consider:   °PTEN testing if head circumference (HCM) is more than 2.5 SD above the mean for age in a child with ID/GDD and/or ASD   °MECP2 testing for Rett syndrome in females with severe ID   ° Karyotype if a chromosomal syndrome is suspected • If other investigations do not provide an etiology and there are unresolved clinical findings, consider ES and mitochondrial DNA testing. Here, FMR1 is linked to Rett syndrome.