Bruton's tyrosine kinase (BTK) also known as tyrosine-protein kinase is an enzyme that plays a role in B-cell maturation in the bone marrow and mast cell activation via the IgE receptor. Mutations in the BTK gene are associated with a rare immunodeficiency disease called Bruton's agammaglobulinemia in which pre-B cell populations develop but fail to mature and enter circulation as antibodies. BTK inhibitors are of interest to pharmaceutical companies as treatments for cancer and inflammatory disorders. Several small molecule inhibitors are currently available for research purposes. The small-molecule BTK inhibitor, Ibrutinib (PCI-32765), was approved by the FDA in 2013 for treatment of B-cell malignancies.