Deficiency in HPRT resulting in less than 1.5% of normal enzyme activity.

More than 200 mutations in the HPRT1 gene have been identified in patients with Lesch-Nyhan Syndrome.

Symptoms of Lesch-Nyhan Syndrome include motor dysfunction that resembles cerebral palsy, variable mental retardation, hypotonia and developmental delay, hyperuricemia with gout, and self-mutilation and persistent self-injurious behaviour.