MECP2 and atypical Rett syndrome: For example, 70% of patients with Rett's syndrome, a pervasive neurological disorder characterized by symptoms like compromised brain functions, and severe mental retardation, have mutations of the MECP2 gene at specific CpG hot spots in exons 3 and 4 (Liyanage & Rastegar, 2014; Petel‐Galil et al., 2006).