Complete or partial loss-of-function mutations of MECP2 lead to Rett syndrome, characterized by a gender-dependent array of symptoms, ranging from early loss of acquired speech and motor skills to severe mental retardation and neonatal encephalopathy, among many others (Amir et al, 1999; Bienvenu & Chelly, 2006). Here, MECP2 is linked to Rett syndrome.