In addition to an array of other neurodevelopmental disorders (Watson et al, 2001; Shibayama et al, 2004; Samaco et al, 2005), MECP2 loss‐of‐function mutations are the most common etiology of RTT (Amir et al, 1999), a severe neurodevelopmental disorder clinically characterized by head growth deceleration, profound cognitive decline, regression of acquired abilities, and stereotypies in early infancy (Hagberg et al, 2002). The gene discussed is MECP2; the disease is neurodevelopmental disorder.