Regulation of CXCR4 and CXCR7 protein expression at the cell membrane using flow cytometry. (A) CXCR4 and CXCR7 expression at the cell membrane of SW480, HCT116, and HT29, in normoxia (20% O2) and hypoxia (1% O 2 ). (B) CXCR4 and CXCR7 expression in SW480 cells after siRNA anti-HIF-1α or anti-CXCR4 in hypoxia. (C) Cell membrane CXCR4 protein expression after transient passage in hypoxia. SW480, HCT116 and HT29 cells were cultured 24 hours in 3% hypoxia and further maintained for 8, 24 and 48 hours in normoxia.