Patients have few therapeutic options following progression on a covalent BTK inhibitor; chimeric antigen receptor (CAR)-T cells are one approach but these are limited to patients with access to tertiary referral centers.56 Retrospective studies demonstrate short OS following progression on ibrutinib, ranging from 2.9 to 5.5 months.57,58 Patients with MCL are also similarly susceptible to the toxicities of non-covalent BTK inhibitors including cytopenias, infections, atrial fibrillation, and bleeding.59 Here, BTK is linked to atrial fibrillation.