MECP2 and atypical Rett syndrome: Its downstream targets encompass ASD-related genes such as BDNF and CDKL5. Mutations in MeCP2 are the major cause of Rett syndrome, a progressive neurodevelopmental disorder with autistic features (Amir et al., 1999; Bienvenu and Chelly, 2006; Chahrour and Zoghbi, 2007).