HCN4 and Arrhythmia: Approximately 2–3 months after tamoxifen‐induced deletion of Ndufs4, 87.5% of the HCN4–Ndufs4−/− mice developed various forms of arrhythmia, including atrial or ventricular premature contractions, sinus bradycardia and frequent sinus pauses (Figure S2B) resembling those found in germline Ndufs4−/− LS mice (Figure 2).