CXCL12 and acute myeloid leukemia: After 2 days (a time point we previously showed was sufficient for MC3T3 cells to acquire the ability to protect AML cells from SDF-1-induced apoptosis) [16], KG1a cells were added to MC3T3 cell cultures for 1 hour, followed by the indicated dose of Ara-C, and the co-cultures were incubated for an additional 16-18 hours.