Rett syndrome (OMIM identifier #312750) is a severe neurological disorder affecting development and function in females, at a rate of 1 in every 10 000 live births.1,2 In the majority of cases (90–95%), patients diagnosed with classic Rett carry loss-of-function mutations in the X-linked methyl-CpG binding protein 2 (MeCP2) gene.1,2 Because of its chromosomal position, random X chromosome inactivation renders female patients somatic mosaics, expressing with varying degrees mutant and normal MeCP2. Here, MECP2 is linked to Rett syndrome.