Hematopoiesis is the cumulative result of intricately regulated signaling pathways that are mediated by cytokines and their receptors. Proper culmination of these diverse pathways forms the basis for an orderly generation of different cell types. Recent studies conducted over the past 10-15 years have revealed that hematopoietic cytokine receptor signaling is largely mediated by a family of tyrosine kinases termed Janus kinases (JAKs) and their downstream transcription factors termed STATs (signal transducers and activators of transcription). Aberration in these pathways, such as that caused by the recently identified JAK2V617F mutation, is an underlying cause for diseases such as leukemias and other myeloproliferative disorders. This recent discovery, when coupled with the fact that STATs are activated by oncoproteins such as BCR-ABL, underscores the importance of the JAK-STAT pathway in both normal cellular development and disease states.