Congenital bile acid synthesis defect type 1 (CBAS1) is a rare inherited disorder that affects the body's ability to produce bile acids. Bile acids are important for digestion and absorption of fats and fat-soluble vitamins. People with CBAS1 have a deficiency of the enzyme 7α-hydroxylase, which is responsible for the production of bile acids. This deficiency leads to an accumulation of bile acids in the body, which can cause a variety of symptoms, including liver disease, poor growth, and neurological problems. Treatment for CBAS1 typically involves dietary changes and supplementation with bile acids.