Lesch-Nyhan syndrome is a rare inherited disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT), produced by mutations in the HPRT gene located on (the) X chromosome.

This results in both hyperuricemia and hyperuricosuria, associated with severe gout and kidney problems. Neurological signs include poor muscle control and moderate mental retardation. These complications usually appear in the first year of life. Beginning in the second year of life, a particularly striking feature of LNS is self-mutilating behaviors, characterised by lip and finger biting. Neurological symptoms include facial grimacing, involuntary writhing, and repetitive movements of the arms and legs.