MECP2 and atypical Rett syndrome: Furthermore, Rac1 appears to be indirectly implicated in the pathogenesis of Rett syndrome, a well-known severe progressive neurodevelopmental condition with significant behavioural abnormalities which is linked to pathogenic variants in MECP2 (Methyl-cpg-binding protein 2, OMIM * 300005) and CDKL5 (Cyclin-dependent kinase-like 5, OMIM * 300203).