U343 coculture induces quiescence in Mer-expressing ALL cell lines. (A) Mer expression in ALL cell lines and in HL-60 cells by western blotting. (B) Culture of leukemic cells alone (−) and coculture of leukemic cells with U343 (+) for 48 hours reduces the number of viable leukemic cells as determined by trypan blue exclusion. (C-H) BrdU incorporation and subsequent 7-AAD staining in leukemic cells cultured alone or in 48-hour cocultures with U343 or MSC cells, as indicated. Cells were gated according to the gating strategy in supplemental Figure 1. Phases of the cell cycle are depicted as indicated. *P < .05; **P < .01; ***P < .001. ns, not significant.