CXCR4 and neoplasm: In the three-line co-culture system, using tumor cells with different CXCR4 expression levels as the target cells, and PBMCs as the effector cells, allowed us to test the effect of knockdown of CXCR4 on the function of trastuzumab-mediated tumor regression by both interrupting HER2 oncogenic signals and ADCC.