As a result of impaired biliary phospholipid secretion and subsequent increase of free nonmicellar bound (potentially toxic) biliary bile acid (BA) concentration, the Mdr2/Abcb4-/- mouse model of sclerosing cholangitis develops pericholangitis, ductular proliferation, reactive cholangiocyte phenotype, and onion skin–type periductal fibrosis,1 reflecting central morphologic features of primary sclerosing cholangitis (PSC).2 The gene discussed is ABCB4; the disease is sclerosing cholangitis.