BTK (Bruton's tyrosine kinase) is a member of the Tec family of kinases, possessing PH, TH, SH3, SH2, and kinase domains. It is predominantly expressed in B cells and certain bone marrow cells, playing a crucial role in B cell development and function. Btk regulates B cell activation, differentiation, and antibody production by participating in signaling pathways of the B cell receptor (BCR) and other receptors. Mutations in the Btk gene, particularly inactivating mutations found in X-linked agammaglobulinemia (XLA), lead to B cell developmental defects and immune deficiencies. Moreover, abnormal activation of Btk in autoimmune diseases and certain cancers has made it a focus of targeted therapy research.