Progressive familial intrahepatic cholestasis (PFIC), originally described as "Byler disease" in an Amish kindred, has been distinguished from other forms of cholestatic liver disease in childhood by clinical findings, clinical-laboratory observations, and morphologic studies in biopsy, hepatectomy, and autopsy specimens. Correlation with genetic analyses has permitted both more precise definition of PFIC and distinctions within PFIC. Two types of PFIC now are recognized: PFIC-1, resulting from mutations in a gene called FIC1 (familial intrahepatic cholestasis, type 1), and PFIC-2, resulting from mutations in a gene called BSEP (bile salt export pump). Other forms of PFIC may yet be identified. The roles of FIC1 and BSEP in the secretion of bile acids into bile and in the post-secretory modification of bile are under study.