Methyl-CpG-bindingprotein 2 (MECP2) is the main causative gene ofRTT; 95% of classical RTT cases were found to be caused by MECP2 pathogenic variants.1 Cyclin-dependent kinase-like 5 (CDKL5) and Forkhead box protein G1 (FOXG1) are responsible for the early seizure variantand congenital variant of atypical RTT, respectively.2,3 However, for a subset of patients with RTTand RTT-like phenotypes, no pathogenic variants have been identified in MECP2, CDKL5, orFOXG1. The gene discussed is MECP2; the disease is Rett syndrome.