CSK and atrial fibrillation: Besides our finding showing that Acalabrutinib exhibits similar effects on Ca handling as Ibrutinib, Acalabrutinib was just recently described as being associated with ventricular arrhythmias after therapy initiation (32), suggesting a functional class effect of these BTK inhibitors with respect to ventricular dysfunction which would be in contrast to the observation by Xiao et al. that Acalabrutinib is not relevant in terms of Ibrutinib/csk-induced AF (27).