Given the pattern of gene body 5mCH in neurons [16, 41] and the role for MeCP2-mediated repression of long genes [100], the Greenberg Lab has proposed a novel hypothesis to address the underlying cause of RTT pathology: as neurodevelopment takes place, neuronal subtypes are specified, and gene body 5mCA is acquired at genes where expression either needs to be repressed or tightly regulated. Here, MECP2 is linked to Rett syndrome.