Bruton's tyrosine kinase (BTK) is a non-receptor tyrosine kinase. It is expressed in all hematopoietic cells, with the exception of T cells and mature plasma cells. Genetic defects of BTK results in X-linked agammaglobulinemia (XLA), a disease in which sufferers cannot generate mature B cells to fight infections. BTK is an essential signaling node downstream of the B cell receptor (BCR) and it is involved in activation of other hematopoietic cells, resulting in cell proliferation, antibody and cytokine production, and co-stimulatory molecule expression. BTK is required for the maturation, function and differentiation of B-cells. BTK is also involved in the signaling pathway of Fcγ receptors (FcγR) for immunoglobulin G (IgG) complex in monocytes and macrophages, as well as the signaling pathway of Fcɛ receptors (FcɛR), which activates mast cells and basophils, resulting in the expression of cytokines, chemokines and cell adhesion molecules.