Sick Sinus Syndrome 2, also known as sinus bradycardia syndrome, familial, autosomal dominant, is related to sick sinus syndrome and atrioventricular block. An important gene associated with Sick Sinus Syndrome 2 is HCN4 (Hyperpolarization Activated Cyclic Nucleotide Gated Potassium Channel 4). Affiliated tissues include heart and heart-atrium, and related phenotypes are mitral valve prolapse and left ventricular hypertrophy