MECP2 and atypical Rett syndrome: Rett Syndrome (RTT, MIM 312750) is a rare, severe neurodevelopmental disorder that primarily affects females [1], occurring in approximately 1:10,000 female births [1, 2] and is primarily (> 96%) due to pathogenic loss-of-function genetic variants of methyl-CpG-binding protein 2 (MECP2) [3].