HPRT1 and Lesch-Nyhan syndrome: In addition to recombinant GLAd delivering the large huntingtin gene, we have successfully constructed recombinant GLAds for many other small-sized transgenes, such as factor IX (R338L Padua mutant, for hemophilia B), glucocerebrosidase (GCR, for Gaucher’s disease), hexosaminidase A (HEXA, for Tay-Sachs disease), hypoxanthine phosphoribosyltransferase 1 (HPRT1, for Lesch-Nyhan syndrome), iduronate-2-sulfatase (IDS, for Hunter syndrome), methyl-CpG-binding protein 2 (MECP2, for Rett syndrome), and survival of motor neuron 1 (SMN1, for spinal muscular atrophy (SMA)).