CXCR4 and cancer: The CXCL12-CXCR4 axis promotes cancer development mainly through two mechanisms: 1) CXCR4-expressing cells are located in CXCL12-expressing organs; and 2) Elevated CXCL12 levels regulate the survival, growth, metastasis and angiogenesis of cancer cells via paracrine signaling [58,59].