Relative to breast cancer cells expressing GFP control, the expression of CXCR4 and, to a greater extent, ACKR3, significantly reduced bioluminescence due to the association between β-arrestin 2 and PKM2 (p < 0.01 and p < 0.005 for CXCR4 and ACKR3, respectively) (Figure 4D,E). This evidence concerns the gene CXCR4 and breast cancer.