Peptidyl arginine deiminase type 4 (PAD-4) is a frequent, specific target in rheumatoid arthritis (RA). A, 35S-methionine–labeled PAD-4 generated by in vitro transcription and translation (IVTT) was subjected to sodium dodecyl sulfate–polyacrylamide gel electrophoresis and visualized by fluorography. B, For each immunoprecipitation, 1 μl of PAD-4 generated by IVTT was mixed with 1 μl of patient serum. Results obtained using sera from 17 patients in the RA pilot group (lanes 1–17) are shown. C, Immunoprecipitation with PAD-4 generated by IVTT was performed as described above, using sera from 16 normal controls (lanes 3–18). The RA sera used in lanes 1 and 2 in B were included as reference sera in the immunoprecipitations shown in lanes 1 and 2 of C.