AbstractPeptidylarginine deiminase type 4 (PAD4/PADI4) posttranslationally converts peptidylarginine to citrulline, in a process known as citrullination. Evidence suggests that PAD4 plays an essential role in pathogenesis of rheumatoid arthritis (RA). RA synovium has many features in common with tumor tissues, including abnormal cell proliferation, extensive fibrin deposition, high coagulation activity, and extreme angiogenesis. The purpose of the present study was to investigate expression of PAD4 in various tumor tissues. Immunohistochemistry indicated that PAD4 had significant expression in many tumor tissues, especially various adenocarcinoma. Western blotting with anti PAD4 antibody and immunostaining with anti citrulline antibody confirmed the expression of the enzyme in these tumors. Furthermore, our immunohistochemistry also detected co‐location of PAD4 with cytokeratin (CK), a well‐known tumor marker for oncological study in many tumors. Western blot analysis also detected citrulline signals in CK extracted from the tumors. In addition, CK 8, 18, and 19 following in vitro citrullination resisted to the digestion of caspase. The results further confirm the expression of PAD4 in the tumors and support that PAD4 may contribute to the disrupted apoptosis of tumors by caspase‐mediated cleavage of CK. Double immunofluorescent labeling detected co‐location of PAD4 with CD34, a cell marker of heamatopoietic progenitor cells (HPC) in bone marrow and other normal tissues, as well as in some fibroblast‐like cells at stroma region of tumors, but not in the tumor cells. The findings imply that PAD4 is initially expressed in CD34+ cells of bone marrow and then distributed in derives of the multi‐potent progenitor cells in diverse tissues. The development of tumor cells expressing PAD4 is possibly associated with abnormal proliferation of CD34+ stem cells. © 2005 Wiley‐Liss, Inc.