Characterization of the antigen defined with monoclonal antibodies (mAbs). (a) Immunoprecipitated proteins by D18 and D19 mAbs were analyzed with SDS‐PAGE in the reducing (left) or non‐reducing (right) condition. (b) MS/MS spectrum of D19‐immunoprecipitated protein ([M + H +] = 1320.65). Peptide sequence was indicated by matching b ion (red) and y ion (blue) fragments. (c) DT40 cells were untreated or subjected to periodate or heat treatment, stained with D18 or D19 mAb, and analyzed by flow cytometry. (d) DT40 cells were stained with fluorescein‐isothiocynate (FITC)‐conjugated D18 or D19 mAb in the absence or presence of excess (100‐folds) inhibitor mAbs, and analyzed by flow cytometry. (e) DT40 cells were stained with D18, D19, or antichicken IgY in the absence or presence of 1% BSA or 30% chicken serum, and analyzed by flow cytometry. Data are shown by mean fluorescence intensity (MFI).