MECP2 and atypical Rett syndrome: These include fragile X syndrome (FXS, caused by loss of function in the FMR1 gene), Rett Syndrome (loss of function in the MECP2 gene), Tuberous Sclerosis Complex (loss of function in either the TSC1 or TSC2 gene), Phelan-McDermid syndrome (PMS, loss of function in the SHANK1, 2, or 3 genes), and Angelman syndrome (loss of function in the UBE3A gene).