Jaundice, also known as icterus, is a yellow pigmentation of the skin which is caused by high blood bilirubin levels (i.e. hyperbilirubinemia). Bilirubin is a natural yellow pigment produce by the breakdown from the red blood cells.

Bilirubin circulates in the blood in the unconjugated form, which is then conjugated in the liver. The conjugation allows the transformation of a water-insoluble molecule in a water-soluble one, from unconjugated bilirubin to conjugated bilirubin, which can then be excreted in the bile.

Crigler-Najjar syndrome is an orphan disease, a rare genetic disorder characterized by elevated levels of bilirubin in the blood (hyperbilirubinemia).

This is due to a deficiency in the enzyme called Glucoronyl-transferase (UGT). UGT catalyzes the hepatic conversion of unconjugated bilirubin into conjugated bilirubin which is the soluble form eliminated from the body.