MECP2 and atypical Rett syndrome: A Rett syndrome model with a germline Mecp2 exon 3 and 4 deletion allele produced healthy mice at birth but male pups (Mecp2–/y null) soon displayed impaired motor skills and premature lethality, while female mice (Mecp2–/+ deficient heterozygotes) became hypoactive and exhibited motor and breathing defects after 3 months of age Post mortem examination of male Mecp2–/y null mice revealed reduced brain and neuronal size (Guy et al., 2001).