Cytokine signaling regulates immune response, hematopoiesis, and cell development. Cytokine receptors, however, lack intrinsic catalytic activity, and thus rely on Janus kinases (Jaks) to mediate signal transduction through the Jak/STAT pathway. Dysregulation of Jak/STAT signaling can lead to myeloproliferative diseases, leukemias, and immunodeficiency. Thus, it becomes important to understand the underlying mechanisms involved in Jak regulation. My project aims to elucidate such mechanisms mainly from a structural standpoint by means of X-Ray Crystallography.