These included 5% with a high resolution chromosomal abnormality, 5% with fragile X syndrome, 5% with Rett syndrome (MECP2 gene defects in females), 3% with PTEN gene mutations in those with a head circumference > 2.5 SD, approximately 10% with other genetic syndromes (e.g., tuberous sclerosis) and 10% with small deletions or duplications identified using chromosomal microarrays. This evidence concerns the gene MECP2 and atypical Rett syndrome.