MECP2 and X-linked syndromic intellectual disability: MECP2-related disorders include classic Rett syndrome (RTT) (MIM 312750), atypical or variant RTT (MIM 312750), severe neonatal encephalopathy (MIM 300673), X-linked intellectual disability (MIM 300055), and MECP2 duplication syndrome or X-Linked intellectual disability syndrome, Lubs type MRXSL (MIM 300260) (Online Mendelian Inheritance in Man, OMIM® n.d.; Vidal et al. 2019; Collins and Neul 2022).