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Type 1, liver kidney microsomal autoantibodies (LKM-1) are associated with a subgroup of idiopathic autoimmune type, chronic active hepatitis (CAH). The antigenic specificity of LKM-1 autoantibodies from 13 patients was investigated by immunoblot analysis of human liver microsomal proteins. Polypeptides of 50, 55, and 64 kD were detected with these antisera. A high titer LKM-1 serum was selected to screen a human liver lambda gt11 cDNA expression library, resulting in the isolation of several complementary (c)DNA clones. Autoantibodies affinity purified from proteins expressed by two of the immunopositive cDNA clones, HLD8.2 and HLD13.2, specifically react with a 50-kD protein of human liver microsomes and display immunofluorescence staining of the proximal renal tubular epithelia characteristic of LKM-1 sera. Determination of the sequence of HLD8.2 revealed that it encodes a recently described cytochrome P450db1. A bacterial fusion protein constructed from HLD8.2 proved to be a specific and sensitive diagnostic reagent. All sera from patients with LKM-1 positive liver disease react with this fusion protein. No reaction was seen, however, for sera from patients with other types of autoimmune liver diseases, viral hepatitis, systemic immunological disorders, or healthy controls.