BTK and mantle cell lymphoma: Ibrutinib, an FDA approved, orally administered small-molecule inhibitor that binds covalently to the C481 residue of Bruton’s tyrosine kinase (BTK), has produced remarkable responses in patients with chronic lymphocytic leukemia (CLL) and many subtypes of non-Hodgkin lymphoma (NHL), including mantle cell lymphoma, Waldenstrom macroglobulinemia, and marginal zone lymphoma.