MECP2 is an X-linked gene whose loss-of-function causes Rett syndrome (RTT; OMIM: 312750) (Amir et al., 1999) and whose duplication causes MECP2 duplication syndrome (MDS; OMIM: 300260) (Lugtenberg et al., 2009; van Esch et al., 2005). The gene discussed is MECP2; the disease is Rett syndrome.