Single gene disorders which include both autosomal and X-linked defects associated with autism as a feature include tuberous sclerosis (TSC1 and TSC2 genes), neurofibromatosis (NF1 and NF2 genes), X-linked Rett (MECP2 gene) and fragile X (FMR1 gene) syndromes. This evidence concerns the gene NF1 and autism.