Serum SS-A/Ro autoantibodies are commonly found in patients with Sjogren's syndrome, systemic lupus erythematosus, neonatal lupus, and subacute cutaneous lupus. Two proteins of 60 and 52 kD have been described as targets for these autoantibodies. To define the 52-kD component unambiguously, cDNA clones were isolated from human HepG2 and MOLT-4 cell cDNA libraries. The identity of cDNA was established by (a) the specificity of the antibody affinity purified from the recombinant protein, (b) the reactivity of the purified recombinant protein with prototype SS-A/Ro sera in immunoblot and ELISA, and (c) two-dimensional gel comigration of MOLT-4 cell 52-kD protein and the recombinant protein. A 1.9-kb cDNA encoded the complete 52-kD protein containing 475 amino acids (Mr 54,082). Putative zinc-finger domains and a leucine zipper motif were identified in the amino-terminal half of the 52-kD protein, implicating its possible association with DNA/RNA. Sequence homology detected between the 52-kD protein and human ret transforming protein, and mouse T cell gene expression down-regulatory protein rpt-1, may provide leads to the functional role of the 52-kD protein in addition to the possibility that these proteins might constitute members of a subfamily of finger proteins.