Forced expression of C/EBPs in combination with ATRA treatment has a synergistic effect on survival of leukemic mice. Leukemic animals expressing either C/EBPϵ-ER (A) or C/EBPα-ER (B) were generated as described in Figure 1 and treated with one of the following: a placebo, 25 mg 4-HT, 10 mg ATRA, or a combination of 4-HT and ATRA. The data for each C/EBP group are from 20 animals with 5 mice per treatment. The combined therapy of 4-HT and ATRA resulted in the most profound effect on survival in both groups (P < .001). The mean survival times of placebo, 4-HT, ATRA, and 4-HT plus ATRA groups were 26.4, 33, 49.6, and 63 days, respectively, in animals receiving transplants with C/EBPϵ-ER–transduced leukemias and 30.5, 41.8, 69, and 97.8 days, respectively, in C/EBPα-ER animals.