Since the MeCP2 gene undergoes X-inactivation, patients are usually female heterozygotes, displaying variable cellular mosaicism for normal and mutant MeCP2.2 Rare cases of hemizygous male patients for mutant MeCP2 have been reported but they rarely survive.2 In mice, MeCP2 loss-of-function is less severe: male hemizygous null mice not only survive until adulthood but they also have been the most commonly studied model system for Rett syndrome, despite the fact that heterozygous female mice better recapitulate the human disease. The gene discussed is MECP2; the disease is Rett syndrome.