CXCL12 and neoplasm: The observed correlation between CXCR4 expression and tumor volume might either be a reflection of decreased oxygen concentrations in larger/ fast-growing tumors resulting in HIF-1α-induced CXCR4 expression, or might be caused by increased tumor cell proliferation via auto- and/or paracrine CXCL12/ CXCR4-mediated growth signalling.