The present disclosure relates to the finding that microRNA-155 plays a role in inflammation, hematopoiesis and myeloproliferation, and that dysregulation of microRNA-155 expression is associated with particular myeloproliferative disorders. Disclosed herein are methods and compositions for diagnosing an treating disorders, including inflammation and myeloproliferation, modulating the levels of expression of one or more genes selected from the group consisting of Cutl1, Arnt1, Picalm, Jarid2, PU.1, Csf1r, HIF1α, Sla, Cebpβ, and Bach1, and the like.