BTK and follicular lymphoma: Ibrutinib, an irreversible inhibitor of Bruton tyrosine kinase (BTK), has become a key molecule in the treatment of chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), Waldenström macroglobulinemia (WM), diffuse large B-cell lymphoma (DLBCL), and follicular lymphoma [6–12].