When 4T1 cells were injected into the mammary glands of wild-type (WT) and Ackr2−/− mice, no difference in primary tumor growth was detected (Fig. 1d), but again the number of spontaneous lung metastasis was significantly lower in Ackr2−/− mice (Fig. 1e, f). This evidence concerns the gene ACKR2 and neoplasm.