The JAK (Janus tyrosine Kinase)-STAT (Signal Transducer and Activator of Transcription) pathway represents one such signaling cascade whose evolutionarily conserved roles include cell proliferation and haematopoiesis. JAK belongs to a family of non-receptor protein tyrosine kinases of approximately 130 kDa, comprising of JAK1, JAK2, JAK3 and TYK2 (non-receptor Protein Tyrosine Kinase-2). STATs are latent cytoplasmic transcription factors that become activated after recruitment to an activated receptor complex. Seven STAT proteins have been identified, STAT1 to 6, including STAT5a and STAT5b, which are encoded by distinct genes. Jaks and Stats are critical components of many cytokine receptor systems; regulating growth, survival, differentiation, and pathogen resistance.