Crigler–Najjar syndrome is a rare inherited disorder affecting the metabolism of bilirubin, a chemical formed from the breakdown of the heme in red blood cells.

[citation needed] Signs and symptoms of Crigler–Najjar syndrome include jaundice, diarrhea, vomiting, fever, confusion, slurred speech, difficulty swallowing, change in gait, staggering, frequent falling and seizures.

Most patients (type IA) have a mutation in one of the common exons (2 to 5), and have difficulties conjugating several additional substrates (several drugs and xenobiotics).

A smaller percentage of patients (type IB) have mutations limited to the bilirubin-specific A1 exon; their conjugation defect is mostly restricted to bilirubin itself.

Before the availability of phototherapy, these children died of kernicterus (bilirubin encephalopathy) or survived until early adulthood with clear neurological impairment.

[1] A San Francisco-based company named Audentes Therapeutics is currently investigating the treatment of Crigler–Najjar syndrome with one of their gene replacement therapy products, AT342.

[10] The homozygous Gunn rat, which lacks the enzyme uridine diphosphate glucuronyltransferase (UDPGT), is an animal model for the study of Crigler–Najjar syndrome.