Btk (Bruton's tyrosine kinase) is a member of the Tec family of protein tyrosine kinases (PTKs) and plays a modulatory role in many cellular processes, such as proliferation, development, differentiation, survival, and apoptosis. The Tec kinases are the second largest family of non-receptor tyrosine kinases and include Tec, Btk, Bmx, Itk, and TXK/Rlk. Mutations of Btk gene cause a primary immunodeficiency disease in humans, X-linked agammaglobulinemia (XLA) which is characterized by a lack of circulating B lymphocytes and an absence of immunoglobulins as a result of defects in B cell maturation and function. Btk is found in all hematopoietic cells, with the exception of T lymphocytes and plasma cells. Btk contains amino-terminal PH (pleckstrin homology) domain which binds phosphatidylinositol (3,4,5)-trisphosphate (PIP3) helping membrane translocation upon PI3 kinase activation. The Tec kinases have similar structure of N-terminal PH domains followed by Tec homology (TH), Src homology 3 (SH3), Src homology 2 (SH2), and kinase domains. Autophosphorylation of Tyr223 in SH3 domain is necessary for full activation of Btk. Various binding proteins have been reported to interact with different domains of Btk.