Janus kinase (JAK) is a type of enzyme that plays a crucial role in the signaling pathways of cells. It helps transmit signals from various growth factors and cytokines, which are important for immune responses and cell growth. JAKs are involved in the regulation of many biological processes, including inflammation and hematopoiesis, the formation of blood cells. There are four main types of JAK enzymes: JAK1, JAK2, JAK3, and TYK2. Each of these has specific functions and is associated with different receptors. Abnormal JAK activity can lead to various diseases, including autoimmune disorders and certain types of cancer, making them important targets for therapeutic interventions.