TRDN (triadin) is a sarcoplasmic reticulum protein essential for cardiac and skeletal muscle excitation-contraction coupling. It regulates calcium release through ryanodine receptors (RYR1/RYR2) and is required for proper organization of the triad junction, where T-tubules contact the sarcoplasmic reticulum 1. In cardiomyocytes, triadin loss impairs calcium handling, decreasing sarcoplasmic reticulum calcium release and slowing L-type calcium channel inactivation, leading to action potential prolongation and cellular arrhythmias including early/delayed afterdepolarizations 1. Triadin knockout syndrome (TKOS) is a rare, malignant inherited arrhythmia disorder caused by biallelic TRDN null mutations 2. Patients present with exercise-induced cardiac arrest or syncope in early childhood (average age 3 years), extensive precordial T-wave inversions, and transient QT prolongation 2. The international registry includes 21 patients; 95% presented with cardiac events and 84% have recurrent breakthrough arrhythmias despite aggressive treatment 2. TRDN was classified as having strong or definitive evidence for causing long QT syndrome with atypical features, including neonatal atrioventricular block 3. Heterozygous carriers show no phenotype. Beyond cardiac manifestations, decreased TRDN expression has been associated with dopaminergic neurodegeneration in Parkinson's disease models 4, suggesting broader physiological roles.