Autism develops as a co-morbid condition in many established single-gene disorders such as fragile X syndrome (MIM 300624), Rett syndrome (MIM 312750), and tuberous sclerosis 1 and 2 (MIM 191100 and MIM 613254), which are caused by mutations in the fragile X mental retardation 1 (FMR1) gene, the methyl CpG binding protein 2 (MECP2) gene, and the tuberous sclerosis 1 (TSC1) and tuberous sclerosis 2 (TSC2) genes, respectively. This evidence concerns the gene MECP2 and atypical Rett syndrome.