ARTICLE ABSTRACTMutational activation of BRAF leading to expression of the BRAFV600E oncoprotein was recently identified in a high percentage of specific hematopoietic neoplasms in monocyte/histiocyte and mature B-cell lineages. Although BRAFV600E is a driver oncoprotein and pharmacologic target in solid tumors such as melanoma, lung, and thyroid cancer, it remains unknown whether BRAFV600E is an appropriate therapeutic target in hematopoietic neoplasms. To address this critical question, we generated a mouse model expressing inducible BRAFV600E in the hematopoietic system, and evaluated the efficacy of pathway-targeted therapeutics against primary hematopoietic cells. In this model, BRAFV600E expression conferred cytokine-independent growth to monocyte/macrophage-lineage progenitors leading to aberrant in vivo and in vitro monocyte/macrophage expansion. Furthermore, transplantation of BRAFV600E-expressing bone marrow cells promoted an in vivo pathology most notable for monocytosis in hematopoietic tissues and visceral organs. In vitro analysis revealed that MAP–ERK kinase inhibition, but not RAF inhibition, effectively suppressed cytokine-independent clonal growth of monocyte/macrophage-lineage progenitors. However, combined RAF and phosphoinositide 3-kinase (PI3K) inhibition effectively inhibited cytokine-independent colony formation, suggesting autocrine PI3K pathway activation. Taken together, these results provide evidence that constitutively activated BRAFV600E drives aberrant proliferation of monocyte-lineage cells.Implications: This study supports the development of pathway-targeted therapeutics in the treatment of BRAFV600E-expressing hematopoietic neoplasms in the monocyte/histiocyte lineage. Mol Cancer Res; 11(12); 1530–41. ©2013 AACR.