NDUFS4 and Bradycardia: As shown in Figure 2, the spectrum of bradyarrhythmia in Ndufs4−/− mice includes frequent sinus node dysfunction (SND, Figure 2A), such as sinus pauses and sinus bradycardia (Figure 2B); episodic atrioventricular (AV) conduction block that manifested as Mobitz type II second-degree AV block (Figure 2C); or rarely, a high-grade AV block (Figure 2D).