It is caused by many different diseases and agents. It is a symptom of several diseases, including Lesch-Nyhan syndrome, phenylketonuria, and Huntington disease and can be a feature of kernicterus (rapidly increasing uncongucated billirubin that cross the blood-brain-barrier in infants).
- "28". Basic and Clinical Pharmacology (Eleventh ed.). McGraw Hill Medical. p. 473. ISBN 978-0-07-160405-5.