BTK and primary central nervous system lymphoma: Recently, the high efficacy of an irreversible selective inhibitor of Bruton’s tyrosine kinase (BTK), ibrutinib, on relapsed/refractory PCNSL has already been reported.2 Tirabrutinib (ONO/GS-4059, Ono Pharmaceutical) is a newly developed another drug for relapsed/refractory PCNSL, which highly selectively inhibits BTK irreversibly.