CXCR4 and cancer: In future studies, it will be important to define whether CXCR4 – RTK feedback among neoplastic cells and with their primary or metastatic stromal microenvironment can confer resistance to plerixafor anti-tumor activities in vivo, and whether such mechanisms apply to the frequent cancer types and are amenable to RTK co-targeting strategies; or whether our observations remain relevant to the interpretation of in vitro cell culture data only.