BTK and mantle cell lymphoma: Thus, Btk represents an important target in the drug development field, with 24 Btk inhibitors (BtkIs) currently under clinical evaluation as anti-tumor agents against chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), B-cell malignancies and mantle cell lymphoma (MCL) in different countries (i.e., USA, China and Poland) [10].