MECP2 and T-B+ severe combined immunodeficiency due to gamma chain deficiency: We assessed integration of transposons encoding wild-type cDNAs (Δexon 1, flanked by a 5′ splice acceptor and 3′ polyA signal) into intron 1 of FANCA (associated with Fanconi anemia), IL2RG (X-linked severe combined immunodeficiency), MECP2 (Rett syndrome), and PAH (phenylketonuria) (Fig. 5H).