PROM1 and neoplasm: In contrast to pancreatic cancer stem cells in which CD133+/CXCR4− cells are just as tumorigenic as CD133+/CXCR4+ cells, in prostate tumor initiating cells CXCR4 expression results in significantly greater colony formation (Figure 2F) and inhibition of CXCR4 with an antagonistic antibody reduces tumor growth (Figure 6D).