CSCs promote tumor angiogenesis; in turn, endothelial cells are able to influence CSC biology and their chemoresistance through the secretion of specific factors, such as stromal cell-derived factor-1α, nitric oxide, angiopoietins, serpins, and the involvement of specific pathways, including Notch, SHH–HEDGEHOG, ABCB1–ABCG2 and ephrins pathways [14,15]. This evidence concerns the gene ABCG2 and neoplasm.