The effect of CCR5 in tumor development was investigated using CCR5+/+ and CCR5−/− mice. B16-F0 melanoma cells were inoculated subcutaneously into CCR5+/+ and CCR5−/− mice (n = 20). Tumor growth was monitored for 31 days. Tumor growth in the CCR5−/− mice was significantly lower than that in the CCR5+/+ mice. The tumor growth in the CCR5−/− mice was reduced to 19.1%, whereas the tumor growth in the CCR5+/+ mice was only reduced to 24.3%, in both tumor weight (Figure 1A and 1B) and tumor volume, respectively (Figure 1A and 1C). These results correlated with mice survival rates. There was a significant difference in survival rates between CCR5−/− mice (75% of starting time) and CCR5+/+ mice (15%) at the end of the experiment (Figure 1D). There was a significant difference in survival rates between CCR5−/− mice and CCR5+/+ mice. At the end of the experiment, 75% of the CCR5−/− mice survived while only 15% of the CCR5+/+ mice survived. Subcutaneous tumors were harvested from the sacrificed mice 31 days after inoculation. The immunohistochemical analysis of tumor sections, stained with H&E and for proliferation antigen PCNA, revealed greater inhibition of tumor cell growth in CCR5−/− mice. The histologic findings after H&E staining indicated that the tumor tissues of the CCR5−/− mice, but not those of the CCR5+/+ mice, contained large areas of necrosis. PCNA reactive cells were significantly reduced in the CCR5−/− mice (Figure 1E).