SCN5A and long QT syndrome 3: Pathogenic and likely pathogenic (P/LP) SCN5A variants impair channel expression and function and underlie a spectrum of disorders, including long QT syndrome type 3 (LQT3), Brugada syndrome type 1 (BrS1), progressive cardiac conduction disorders (PCCD), sick sinus syndrome (SSS), atrial standstill, familial atrial fibrillation, and multifocal ectopic Purkinje-related premature contractions (MEPPC).