Ibrutinib is a first-in-class, once-daily, oral inhibitor of Bruton tyrosine kinase (BTK) approved for the therapy of B cell malignancies including chronic lymphocytic leukemia (CLL), Waldenström macroglobulinemia (WM), marginal zone (MZL), and mantle cell lymphoma (MCL). The gene discussed is BTK; the disease is mantle cell lymphoma.