Rett syndrome (RTT, OMIM #312750) is mainly caused by loss-of-function mutations in MECP2, and MECP2 duplication syndrome (MDS, OMIM #300260) is associated with duplications in the Xq28 locus, with a minimal region of duplication containing MECP2 and the adjacent gene Interleukin-1 receptor-associated kinase 1 (IRAK1) [6]. The gene discussed is MECP2; the disease is Rett syndrome.