OBJECTIVE To study the effects of Duffy antigen receptor for chemokines (DARC) on the tumorigenesis and metastasis of breast cancer. METHODS Human breast cancer cells of the line MDA-MB-435HM with a great potentiality of metastasis were cultured and then divided into 3 groups: MDA-MB-435HM-DARC cell group, to be transfected with pcDNA3/DARC FyB containing human DARC cDNA; MDA-MB-435HM-vect cell group, to be transfected with blank plasmid pcDNA3; and MDA-MB-435HM cell group without transfection. Female BALB/c nude mice were implanted with MDA-MB-435HM cells into the nipple. The size of the implanted cancer was measured once a week. The mice were killed 4, 6, and 8 weeks after the implantation respectively and their lungs were taken out to observe the number of metastatic tumor. Immunohistochemical staining was performed to calculate the microvascular density (MVD). Western blotting was used to detect the matrix metalloproteinase (MMP)-9 protein expression of the lung tissues. ELISA was used to detect the concentrations of the 2 DARC ligands: CNCL8 and CCL2 in the supernatant of culture fluid of the 3 kinds of cells and the metastatic tumors. RESULTS The expression levels of DARC mRNA and protein of the MDA-MB-435HM cells were 38% those of the MDA-MB-435 cells. The expression levels of CXCL8 and CCL2 of the MDA-MB-435HM-DARC cells were significantly lower than those of the MDA-MB-435HM-vect cells and MDA-MB-435HM cells (both P < 0.05). All mice developed tumor with a tumorigenesis rate of 100%. However, the tumor occurred 17 days after implantation in those mice implanted with MDA-MB-435HM-DARC cells, significantly later then in those implanted with MDA-MB-435HM-vect cells and MDA-MB-435HM cells (9 and 10 days after implantation) the size of tumor in the former group being significantly smaller than those of the 2 latter groups (both P < 0.01). The number of metastatic foci in the lung of the MDA-MB-435HM-DARC group was significantly less than those of the other 2 groups (both P < 0.01). The level of CCI2 of the implanted tumor in the MDA-MB-435HM-DARC group was significantly lower than those of the other 2 groups (both P < 0.01). The MVD of the MDA-MB-435HM-DARC was significantly less than those of the other 2 groups (both P < 0.01) and most of the vessels of the MDA-MB-435HM-DARC group were not newly formed vessels with a diameter < 8 red blood cells. The MMP-9 protein expression levels of the MDA-MB-435HM-vect cells and MDA-MB-435HM-cells were 3.1 and 3.4 times that of the MDA-MB-435HM-DARC group (both P < 0.05). CONCLUSION DARC, a chemokine decoy receptor, inhibits the chemokine-induced angiogenesis, thus inhibiting the growth and lung metastasis of breast cancer.