CXCR4 and cancer: In particular, CXCR4-mediated interaction that holds cancer (re-)initiating cells within a protective tumor microenvironment (TME) seems to be responsible for resistance to pharmacological treatment, and for relapse, at least in hematopoietic cancers (Teicher & Fricker, 2010; Mendelson & Frenette, 2014; Shain & Tao, 2014).