For example, thousands of young girls have Rett Syndrome, a neurodevelopmental disorder caused by a mutation on the X-linked gene MECP2 (Bianchi et al., 2012; Lee and Bartolomei, 2013; Carrette et al., 2017; Gribnau and Barakat, 2017). The gene discussed is MECP2; the disease is atypical Rett syndrome.