BTK and plasma cell myeloma: Using a fluorescence-labeled derivative of ibrutinib, we here demonstrate that indeed ibrutinib a non-cytotoxic concentration of 1 μM not only eliminates BTK activity in MM cells, but specifically targets also an unidentified 64-kD polypeptide that is present also in bortezomib-resistant myeloma cells that lacked detectable BTK expression but were sensitive to the combination ibrutinib + LU-102.