BTK and atrial fibrillation: While ibrutinibpossesses high affinity toward BTK, it lacks selectivity within thesubset of kinases that possess a cysteine residue in a similar region.6 Off-target inhibition of these kinases, includingITK, TEC, BLK, EGFR, and JAK3, can lead to adverse events in patientssuch as rash, atrial fibrillation, and major hemorrhage.13−15 These side effects have largely prevented BTK inhibitors from beingapproved as therapeutics for autoimmune diseases.