MECP2 and atypical Rett syndrome: BDNF has been implicated in Rett syndrome (RTT), an X-linked neurological disorder caused by loss-of-function mutations in the transcriptional modulator methyl-CpG-binding protein 2 (MECP2; Amir et al., 1999; Percy and Lane, 2005; Bienvenu and Chelly, 2006; Chahrour and Zoghbi, 2007).