Forced expression of PU.1 at WT levels in promyelocytic leukemia cells was demonstrated to inhibit clonogenic growth, force monocytic differentiation, and induce apoptosis by Cook et al. These findings support the hypothesis that the suboptimal expression of PU.1 can be a key event in the permission of leukemogenesis by blocking proper maturation of the cell [85,91]. Here, SPI1 is linked to acute promyelocytic leukemia.