CXCR4 and breast carcinoma: Exposure of two human breast cancer cell lines MDA-MB 231 and MCF7 to hypoxia led to a statistically significant increase in cell surface expression of CXCR4 by 27.5% (p < 0.05) and 67.5% (p < 0.05), respectively, when compared to cells exposed to normoxia (Figure 2A-C).