Crigler-Najjar (CN) syndrome is a very rare inherited disorder caused by deficiency of the bilirubin uridine diphosphate glucuronosyl transferase (bilirubin-UGT) enzyme. Deficiency of the bilirubin-UGT enzyme results in high levels of a toxic form of bilirubin, a product of the breakdown of red blood cells. High levels of this bilirubin can cause kernicterus, a form of brain damage, which could lead to hearing problems or intellectual disability. Symptoms of affected children include yellowing of the skin, lack of energy, low muscle tone, and occasionally increased tightness of muscles. Symptoms are usually evident at birth or first weeks of life. There are two types of CN syndrome; CN1 and CN2. CN1 is very severe and death usually occurs in childhood. CN2 is less severe and affected individuals can generally survive into adulthood without any intellectual impairment. CN is caused by pathogenic variants in the UGT1A1 gene.