The significance of this particular factor is supported by the fact that, 2–4 months prior to RI-AML onset, those 10%–30% of RJL/L mice that will develop solely radiation-induced cancer have significantly elevated CSF-1 levels as compared to those mice in which RI-AML fails to develop or those that develop RCN B. The observation that RI-AML cells in vitro synthesize significant amounts of CSF-1 further supports the hypothesis that CSF-1 is necessary for leukemia progression [49]. This evidence concerns the gene CSF1 and acute myeloid leukemia.