Striatal function is severely perturbed in prominent disorders like Huntington disease (HD), as well as in less common ones such as Lesch-Nyhan syndrome (LNS), a neurodevelopmental disorder caused by mutations in the gene encoding the purine reutilization enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT). The gene discussed is HPRT1; the disease is juvenile Huntington disease.