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A mutation of the UGT1A1 gene causes Crigler-Najjar syndrome. The UGT1A1 gene creates enzymes in the liver that break down bilirubin to remove it from your body.

Bilirubin is a yellow substance that forms when red blood cells reach the end of their life cycle. Bilirubin takes on two forms in your body. Unconjugated bilirubin is a toxic substance that forms from red blood cells. Your liver takes unconjugated bilirubin and breaks it down into conjugated bilirubin, the nontoxic form of the substance. Once bilirubin is in its nontoxic form, your body gets rid of it when you poop.

If you have a mutation on the UGT1A1 gene, your body can’t break down bilirubin, so its toxic form collects in your blood and tissues. If you have a toxic substance in your blood, it can cause life-threatening symptoms of Crigler-Najjar syndrome if left untreated.