Although we did not observe a difference in the cycle status of leukemia cells upon disease development in Cxcl12−/− mice, given that CXCL12 keeps normal HSCs in a quiescent state (Tzeng et al., 2011), we speculate that CXCL12 also restrains cell cycle progression of AML cells at the early stages of AML development. The gene discussed is CXCL12; the disease is leukemia.