BTK and mantle cell lymphoma: Shortly after the FDA approval of afatinib, ibrutinib, a covalent inhibitor of Bruton's tyrosine kinase (BTK), was first approved by the FDA in 2013 for the treatment of mantle cell lymphoma (MCL) and later approved for the treatment of chronic lymphocytic leukemia (CLL) and Waldenström macroglobulinemia (Figure 1A) [17].