Both CXCR4 and CXCR7 are essential for cell migration. a Wild-type (W) and each HeLa cell clone lacking the receptors (7KO #1,2,3 and 4KO #1,2) were seeded into 96-well plates. The CCK-8 assay was performed using different plates each day. Values are means ± SDs. b, c Migration assay using HeLa cells. Wild-type (W) and knock-out cells infected with CXCR7 (7KO/ CXCR7) or CXCR4 (4KO/ CXCR4) were placed in the upper wells of transwell plates. 50 ng/ml of SDF-1α was added to serum-free media in the lower wells (NT: no treatment). After 24 h, migrated cells in the lower wells were stained and counted under an inverted microscope. c The average number of migrated cells (four different microscopic fields) is shown for the five groups. d Migration assay with U937 cells. CXCR7 knock-out U937 cells were re-introduced CXCR7 by infection (V: empty plasmid). Cells were placed in the upper wells, with the lower well containing increasing concentration of SDF-1α in serum-free medium. After 6 h, migrated cells into the media of the bottom wells were counted using a hemocytometer. Cell numbers are averages of migrated cells from three different wells. Results are the average of three independent experiments. Values are presented as the mean ± SD. **p < 0.01, **p < 0.001.