Bile Acid Synthesis Defect, Congenital, 2, also known as CBAS2 or cholestasis with delta -3-oxosteroid 5-beta-reductase deficiency, is an autosomal recessive anomaly of bile acid synthesis, characterized by severe and rapidly progressive cholestatic liver disease, and malabsorption of fat and fat-soluble vitamins. Patients with this disease show absence or low levels of chenodeoxycholic acid and cholic acid in plasma and urine.

Diagnosis of CBAS2 is based on urine analysis using liquid secondary ionization mass spectrometry (LSIMS) and gas chromatography - mass spectrometry (GC-MS). Treatment is based on oral bile acid therapy. Proper management of the condition leads to gradual resolution of biochemical and histologic abnormalities and prevents progression of the disease.

Mutations in the AKR1D1 gene have been found in patients with CBAS2. The AKR1D1 gene encodes an enzyme called 3-oxo-5-beta-steroid 4-dehydrogenase, which catalyzes the bile acid intermediates 7-alpha,12-alpha-dihydroxy-4-cholesten-3-one and 7-alpha-hydroxy-4-cholesten-3-one. Chenodeoxycholic acid and cholic acid are decreased in plasma and urine.