Bilirubin is a yellow bile pigment that is a degradation product of heme. It occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the destruction of aged or abnormal red blood cells. Bilirubin has been found in all vertebrates and in certain plants including Strelitzia nicolai (PMID: 28573242). Bilirubin levels in humans are elevated in certain diseases such as jaundice and liver disease and it is responsible for the yellow color of bruises and the yellow discoloration in jaundice. Bilirubin breakdown products, such as stercobilin, cause the brown color of feces. A different breakdown product, urobilin, is the main component of the straw-yellow color in urine. Bilirubin consists of an open chain of four pyrroles (tetrapyrrole). It is formed by oxidative cleavage of a porphyrin in heme, which leads to biliverdin, a green tetrapyrrolic bile pigment that is also a product of heme catabolism. Biliverdin is then reduced to bilirubin via biliverdin reductase. After conjugation with glucuronic acid, bilirubin can be excreted in the urine. Bilirubin is structurally similar to the pigment phycobilin used by certain algae to capture light energy, and to the pigment phytochrome used by plants to sense light. Elevated bilirubin levels in humans are associated with Crigler-Najjar syndrome type I, which is an inborn error of metabolism. Crigler-Najjar syndrome is a rare genetic disorder characterized by an inability to properly convert and clear bilirubin from the body. Affected individuals cannot convert unconjugated bilirubin to the conjugated form because they lack a specific liver enzyme required to break down (metabolize) bilirubin. Since they cannot convert bilirubin, they develop abnormally high levels of unconjugated bilirubin in the blood (hyperbilirubinemia). Crigler-Najjar syndrome is caused by mutations in the UGT1A1 gene. The hallmark finding of Crigler-Najjar syndrome is a persistent yellowing of the skin, mucous membranes and whites of the eyes (jaundice). Elevation of both alanine aminotransferase and bilirubin levels in serum or plasma can be indicative of serious liver injury. High levels of bilirubin are indicative of jaundice, which is easily recognizable due to a yellowing of the skin and eyes.  Bilirubin is also an antioxidant. Bilirubin's antioxidant activity may be particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging superoxide during N-methyl-D-aspartic acid neurotransmission (PMID: 31353321).