Ibrutinib, sold under the brand name Imbruvica among others, is a small molecule drug that inhibits B-cell proliferation and survival by irreversibly binding the protein Bruton's tyrosine kinase . Blocking BTK inhibits the B-cell receptor pathway, which is often aberrantly active in B cell cancers. Ibrutinib is therefore used to treat such cancers, including mantle cell lymphoma, chronic lymphocytic leukemia, and Waldenström's macroglobulinemia. Ibrutinib also binds to C-terminal Src Kinases. These are off-target receptors for the BTK inhibitor. Ibrutinib binds to these receptors and inhibits the kinase from promoting cell differentiation and growth. This leads to many different side effects like left atrial enlargement and atrial fibrillation during the treatment of Chronic Lymphocytic Leukemia.