MECP2 and neurodevelopmental disorder: Rett syndrome (RTT) is a rare, X-linked, severe neurodevelopmental disorder, predominantly associated with pathogenic variants in the methyl-CpG-binding protein-2 (MECP2) gene, with an increasing number of atypical RTT or RTT-like individuals having pathogenic variants in other genes, such as cyclin-dependent kinase-like 5 (CDKL5) or forkhead box G1 (FOXG1).