NPC1 and Rett syndrome: Prior single gene testing among this sample included Fragile-X (FMR1), Ataxia-Telengectasia (ATM), Niemann-Pick C (NPC1, NPC2), spinal muscular atrophy (SMN1, SMN2), Prader-Willi syndrome (15q11.2-q13), myotonic dystrophy (DMPK), ARX, atypical Rett syndrome (CDKL5), and Glutaric aciduria Type 1 (GAT1).