HCN4 and sick sinus syndrome: While mutations in the cardiac sodium channel α-subunit encoding gene (SCN5A) (Nof et al., 2007; Tan et al., 2001) and hyperpolarization-activated cyclic nucleotide-aged channel encoding gene (HCN4) Schulze-Bahr et al., 2003; Verkerk and Wilders, 2015 have been found to cause SSS, only a few other genes affecting the structure and/or function of the SAN were identified to increase the risk of developing SSS (Anderson and Benson, 2010; Holm et al., 2011).