Colorectal cancer is the third leading cause of cancer deaths in American men and women. We describe the cytotoxic use of HIV-1 Nef protein and a cytotoxic peptide identified within the HIV-1 Nef structure in targeting human cancer cells in vitro and in vivo in a human xenograft model. A human colorectal tumor was implanted and propagated in the subcutaneous tissue of SCID mice. The mice were injected biweekly with the Nef apoptotic peptide. The tumor treated with Nef peptide underwent significant growth inhibition by as much as 300 percent when compared to the control (untreated) tumors. The Nef peptides were found to have an apoptotic effect on the human colon tumor similar to the effect seen on CD4 cells when the viral protein is secreted by the HIV-1 virus infected cells. The evidence from the xenograft mouse model suggests that the Nef peptides can be used to inhibit human colorectal cancer growth.