Crigler-Najjar syndrome is caused by an abnormal gene which fails to produce a functional enzyme (bilirubin glucuronyltransferase) capable of converting bilirubin into a water-soluble and therefore, easily excreted form. As a result, bilirubin can build up in the body, which can damage the brain and other organs. The syndrome is inherited as an autosomal recessive trait. This means that the child must get the defective gene from both parents to develop the severe form of the condition. Parents who are carriers (with just one defective gene) have about half the enzyme activity of a normal adult. Infants who inherit the trait from both parents (this is called being homozygous for the abnormal gene) develop severe jaundice (hyperbilirubinemia) beginning a few days after birth. If these infants are not treated, they may develop kernicterus, which is bilirubin toxicity of the brain and can be fatal. In such infants, the jaundice will persist into adult life and may require daily treatment. The constantly elevated levels of bilirubin may eventually produce an adult form of kernicterus despite treatment. If left untreated, this severe infant-onset form of the disease will lead to death in childhood. Milder forms of the disease (type II) are not associated with severe toxicity, liver damage, or changes in thinking during childhood. Affected individuals still have jaundice, but they have fewer symptoms and less organ damage.
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