MeCP2 provides an interesting case in which the available evidence indicates that loss of MeCP2 (Rett Syndrome) can produce an “undergrowth” entity, while excess MeCP2 (MeCP2 duplication syndrome) conversely results in a presentation consistent with underlying “overgrowth.” Rett Syndrome (RTT), in 95% of cases, is caused by loss-of-function mutations in the X-linked MeCP2 and is predominately seen in females (around 1/10,000 live births) as these mutations in males lead to perinatal lethality (Amir et al., 1999). This evidence concerns the gene MECP2 and Rett syndrome.