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The novel gene H731 (approved name: PDCD4 (programmed cell death 4)) has been isolated as an antigen gene of the monoclonal antibody Pr-28 which recognized a nuclear antigen in proliferating cells. The gene is homologous to the mouse gene (MA-3/Pdcd4/A7-1) which was associated with apoptosis and was shown to suppress tumor promoter-induced neoplastic transformation. A polyclonal antibody against H731-protein derived from an extract of Escherichia coli transformed with an H731 expression plasmid was prepared, and the H731-protein expression in human normal and tumor cells using the antibody was studied. The staining patterns of asynchronous cultures of human normal fibroblasts (MRC-5) were heterogeneous but the antigen was accumulated in the nuclei at the G0 phase. On the contrary, the antigen was overproduced and localized in the cytoplasm during the cell cycle in tumor cell lines. Immunohistological studies revealed that the H731-protein was highly expressed in bladder carcinoma and breast carcinoma tissues compared with the normal tissues so far tested. These results indicated that expression of the H731-protein was up-regulated or induced in the proliferative cells. Immunohistological studies also revealed that the protein was abundantly expressed in the small duct epithelial cells of the normal mammary gland.