A high tumor expression of CXCL12 was defined as the positive tumor expression of CXCL12 in five studies [11–14, 17], the presence of CXCL12 in > 5% [19] or > 30% [15] of cancer cells in two studies, and the presence of CXCL12 above median density value in another three studies [16, 18, 20]. This evidence concerns the gene CXCL12 and cancer.