MECP2 and atypical Rett syndrome: We also identified variants in genes that are linked to neurological syndromes, such as MECP2, CDKL5, GRIN2A, SCN1A, PCDH19, UBE3A, and SLC9A6. Developmental regression is frequently seen in patients with Rett Syndrome, Landau Kleffner Syndrome, and Christianson Syndrome.