ASD often co-occurs with rare Mendelian neurodevelopmental disorders such as fragile X syndrome (FXS) [131, 132], Rett syndrome (RTT) [133, 134], neurofibromatosis type 1 (NF1) [135], and tuberous sclerosis complex (TSC) [136] as well as copy number variations like those in Phelan–McDermid syndrome, which involves mutations in SH3 and multiple ankyrin repeat domains 3 (SHANK3) [137]. The gene discussed is TSC1; the disease is Rett syndrome.