Macroscopic and microscopic effects of INCB018424 on spleens from mice inoculated with Ba/F3-EpoR-JAK2V617F cells. Massive splenomegaly is observed in vehicle-treated mice as assessed by gross appearance (A top) or hematoxylin and eosin-stained transverse sections collected at the midline (A bottom), although markedly reduced with INCB018424 treatment. (B) Microscopic examination (original magnification ×20) of tissue sections shows the massive accumulation of malignant cells in the vehicle group with an abundance of mitotic figures and multinucleated cells present. Treatment with INCB018424 dramatically reduces the malignant cell burden and restores normal lymphoid architecture.