BTK and mantle cell lymphoma: In the last decade or so, the finding that BTK is overexpressed/hyperactive in some B-cell malignancies led to the rapid development and approval of Ibrutinib (Imbruvica), the first BTK inhibitor, for treating patients with relapsed/refractory chronic lymphocytic leukemia and mantle cell lymphoma (MCL) (Cameron and Sanford, 2014).