Bruton's tyrosine kinase (BTK) is a non-receptor tyrosine kinase that plays a crucial role in B-cell development and function. It is encoded by the BTK gene and consists of five distinct structural domains, including a kinase domain. BTK is essential for transmitting signals from B-cell antigen receptors, leading to processes like calcium mobilization and B-cell maturation. Mutations in the BTK gene can cause X-linked agammaglobulinemia (XLA), an immunodeficiency characterized by a lack of mature B cells and antibodies. In addition to its role in B cells, BTK is also involved in mast cell activation and has been implicated in various cancers, where its abnormal activation can lead to uncontrolled B-cell proliferation. Due to its involvement in B-cell signaling and cancer, BTK has become an important target for therapeutic interventions.