Variants in MECP2 underlie two pediatric neurodevelopmental disorders: Rett syndrome (RTT), caused by MECP2 loss-of-function variants (Amir et al., 1999), and MECP2 duplication syndrome (MDS), resulting from duplications of the X chromosome region Xq28 (Van Esch et al., 2005). The gene discussed is MECP2; the disease is neurodevelopmental disorder.