Gilbert syndrome is a mild liver disorder that impairs the body's ability to process bilirubin, a substance made when old red blood cells are broken down. This leads to fluctuating levels of bilirubin in the blood, sometimes causing levels to be high (hyperbilirubinemia). Most people with Gilbert syndrome do not have symptoms or have mild jaundice. In some cases, jaundice is triggered or made worse by stress, exercise, fasting, dehydration, drinking alcohol, or illness. Some people with Gilbert syndrome have reported other symptoms such as fatigue, weakness, nausea, or diarrhea. There has not been evidence these other symptoms are caused by hypebilirubinemia, and the cause of these symptoms currently is unclear. People with Gilbert syndrome may also have more side effects from certain drugs such as irinotecan. Gilbert syndrome is caused by genetic changes in the UGT1A1 gene and inheritance is autosomal recessive. Genetic changes in this gene cause reduced levels of a liver enzyme needed to eliminate bilirubin from the body, causing bilirubin to accumulate. People with Gilbert syndrome have about one third of the normal enzyme activity, which usually is enough to prevent symptoms from developing. Of note, genetic changes in the UGT1A1 gene can alternatively cause other disorders, such as Crigler-Najjar syndrome. There are two forms: Crigler-Najjar syndrome type 1 (CN-1) and Crigler-Najjar syndrome type 2 (CN-2). In both types, jaundice is persistent and more severe than in Gilbert syndrome, with CN-1 causing potentially severe symptoms. Sometimes it can be hard to distinguish between Gilbert syndrome and CN-2 because of considerable overlap in measured bilirubin levels. Genetic testing to identify the specific genetic change present is sometimes needed for the correct diagnosis.