JAK tyrosine kinases and STAT transcription factors constitute a signaling pathway, which is activated by cytokines. By activating gene transcription it regulates essential biological responses to environmental cues. The Jak-Stat pathway is involved in the regulation of cell development, differentiation, proliferation and apoptosis. Improper function may contribute to hematopoietic malignancies and cancer. This book provides comprehensive insights into the latest basic and cl