Updated project metadata. To uncover novel epigenetic regulators in AML, we performed an in vivo short hairpin RNA (shRNA) screen in the context of Cebpa mutant AML. This led to the identification of the Histone 3 Lysine 4 (H3K4) demethylase, KDM5C, as a novel tumor suppressor in AML. KDM5C potentially functions as a transcriptional repressor via its demethylase activity at promoters, and dysregulation could therefore have widespread consequences. Here, we found that reduced Kdm5c/KDM5C expression is associated with accelerated growth in both human and murine AML cell lines. In vivo, Kdm5c knockdown in a Cebpa mutant AML mouse model resulted in a more aggressive, immature and short-latency phenotype. Mechanistically, we show that knockdown of Kdm5c increased H3K4me3 globally. This translated into the up-regulation of a group of bivalently marked immature genes, resulting in a de-differentiation phenotype which could be reversed by modulating levels of pro-differentiation factors. Finally, we demonstrated that low levels of KDM5C were associated with a decrease in long-term disease-free survival, specifically in female patients. This emphasizes the clinical relevance of our findings and identifies KDM5C as a novel female-biased tumor suppressor in AML.