CXCR4 and cancer: Interestingly, however, inhibition of CXCR4 signaling has recently been reported to interfere with responses of protumorigenic neutrophils in a zebrafish xenotransplantation model of breast cancer40 as well as of immunosuppressive Ly6Clow monocytes in experimental colorectal cancer,41 pointing to distinct roles of this chemokine receptor, which can also be expressed by tumor cells, in different types of cancer.