HCN4 and Sinus bradycardia: In our case, the results of the deep in silico analysis of HCN4 p.V642M strongly suggest the deleterious effect of this variant on the protein, and the presence of severe bradycardia in the proband’s sister, who is genotype-negative for LQTS type 3 and harbors only the HCN4 p.V642M substitution, testifies to the causal role of this variant in sinus bradycardia.