The BTK inhibitor ibrutinib (PCI‐32765, Imbruvica) is a Food and Drug Administration‐approved drug for the treatment of a number of B cell cancers, including mantle cell lymphoma, chronic lymphocytic leukemia/small lymphocytic lymphoma, Waldenström's macroglobulinemia and marginal zone lymphoma (Table 3). This evidence concerns the gene BTK and B-cell chronic lymphocytic leukemia.