CXCR4 and breast cancer: This hypothesis was developed from three observations: 1) the utilization of CXCR4 as the coreceptor for HIV X4 and R5/X4 strains [1], 2) the common and often high level expression of CXCR4 in breast neoplasia [4]–[6], and 3) the induction of apoptosis of breast cancer cells by the specific binding of CXCR4-tropic HIV envelope to CXCR4 [10].