It has been hypothesized that inborn errors of ileal bile acid transport result in life-threatening diarrhea in infancy and lowered serum cholesterol with increasing age in childhood, adolescence and adulthood. Cloning of the ileal bile acid transport gene allows opportunities to evaluate subjects with congenital bile acid diarrhea for evidence of mutations. One subject has been evaluated and found to have three mutations resulting in non-expression of the transporter in a test cell system.