Bruton's tyrosine kinase (or BTK) is a type of kinase protein expressed in B lymphocytes and T cells. BTK contains a PH domain which binds phosphatidylinositol(3,4,5)-trisphosphate (PIP3). After binding to PIP3, BTK is induced to phosphorylate phospholipase C, which in turn hydrolyzes PIP2 into two second messagers, IP3 and DAG, which then modulate the activity of downstream proteins during B-cell signaling. Btk is also found implicated in the primary immunodeficiency disease X-linked agammaglobulinemia(Bruton's agammaglobulinemia). BTK played a key role in B-cell maturation as well as mast cell activation through the high-affinity IgE receptor. Patients with X-linked agammaglobulinemia have normal pre-B cell populations in their bone marrow but these B-cells can not mature and enter the circulation.