KCND3 encodes Kv4.3, a voltage-gated potassium channel subunit that forms A-type potassium channels mediating transient outward potassium currents in excitable tissues 1. The channel exhibits characteristic fast activation at subthreshold membrane potentials, rapid inactivation, and quick recovery from inactivation 1. In cardiac muscle, KCND3 contributes to ventricular repolarization and may generate the transient outward potassium current I(To), with genetic variants associated with early repolarization patterns on ECG and Brugada syndrome 23. In the nervous system, the channel conducts transient subthreshold somatodendritic A-type potassium currents involved in neuronal excitability regulation 4. KCND3 mutations cause spinocerebellar ataxia types 19/22 (SCA19/22) through dominant-negative effects, where mutant channels exhibit loss-of-function phenotypes, enhanced protein degradation, and defective membrane trafficking that disrupts wild-type channel function 1. The clinical spectrum includes both early-onset neurodevelopmental disorders with epilepsy and late-onset ataxic syndromes 45. Recent studies have identified KCND3 variants in childhood epileptic encephalopathies, expanding the recognized phenotypic spectrum beyond traditional cardiac and cerebellar presentations 5.