Because the results mentioned above suggest that CXCR4 is implicated in breast cancer dormancy in both the PDX and cell line-derived tumor models, and the cancer cells in the cell line-derived tumor model grew faster and metastasized more efficiently than those in the PDX model, we hereafter examined using the MDA-MB-231 cell line-derived tumor model. This evidence concerns the gene CXCR4 and cancer.