Although much has been learned about the molecular basis of immunoglobulin M (IgM) rheumatoid factors (RFs) in healthy individuals and in patients with mixed cryoglobulinemia and rheumatoid arthritis, little is known about the genetic origins of the potentially pathogenic IgG RFs in the inflamed rheumatoid synovia of patients. Recently, we generated from unmanipulated synovium B cells several hybridomas that secreted self-associating IgG RFs. To delineate the genetic origins of such potentially pathogenic RFs, we adapted the anchored polymerase chain reaction to rapidly clone and characterize the expressed Ig V genes for the L1 and the D1 IgG RFs. Then, we identified the germline counterparts of the expressed L1 IgG RF V genes. The results showed that the L1 heavy chain was encoded by a Vh gene that is expressed preferentially during early ontogenic development, and that is probably located within 240 kb upstream of the Jh locus. The overlap between this RF Vh gene and the restricted fetal antibody repertoire is reminiscent of the natural antibody-associated Vh genes, and suggests that at least part of the "potential pathogenic" IgG RFs in rheumatoid synovium may derive from the "physiological" natural antibody repertoire in a normal immune system. Indeed, the corresponding germline Vh gene for L1 encodes the heavy chain of an IgM RF found in a 19-wk-old fetal spleen. Furthermore, the comparisons of the expressed RF V genes and their germline counterparts reveal that the L1 heavy and light chain variable regions had, respectively, 16 and 7 somatic mutations, which resulted in eight and four amino acid changes. Strikingly, all eight mutations in the complementarity determining regions of the V gene-encoded regions were replacement changes, while only 6 of 11 mutations in the framework regions caused amino acid changes. Combined with L1's high binding affinity toward the Fc fragment, these results suggest strongly that the L1 IgG RF must have been driven by the Fc antigen.