High levels of toxic bilirubin in your blood cause Crigler-Najjar syndrome, a rare genetic condition. Bilirubin is a substance that forms when red blood cells reach the end of their lifespan. Your liver breaks down bilirubin from a toxic substance to a nontoxic substance that you get rid of in your stool. If you’re diagnosed with Crigler-Najjar syndrome, your liver can’t break down bilirubin and it collects in your blood. As a result, Crigler-Najjar syndrome can cause life-threatening symptoms if left untreated.