Tumor cells were originally thought to be the primary source of MCP-1 in established tumors (4–6); however, recent studies indicated that stromal cells were the primary cell source of MCP-1 in some mouse tumor transplantation models, including 4T1 breast cancer (23), M5076 sarcoma, and B16 melanoma (27). The gene discussed is CCL2; the disease is breast carcinoma.