BTK and mantle cell lymphoma: Given the central role of BTK in B‐cell diseases and its relatively restricted expression, BTK inhibition has emerged as an attractive and promising way to treat multiple relapsed lymphoproliferative syndromes, ranging from chronic lymphocytic leukaemia (CLL) to mantle cell lymphoma (MCL), Waldenström macroglobulinaemia (WM) and, more recently, marginal zone lymphoma (MZL).2