Ibrutinib (IBT) is an irreversible inhibitor of Bruton’s Tyrosine Kinase (BTK) that was initially developed and approved to treat B-cell malignancies such as Mantle Cell Lymphoma (MCL) and Chronic Lymphocytic Leukemia (CLL)43–45. The gene discussed is BTK; the disease is B-cell chronic lymphocytic leukemia.