KCND1 encodes Kv4.1, an A-type voltage-gated potassium channel subunit that plays critical roles in neuronal excitability and development 1. The channel mediates rapidly inactivating A-type currents in neurons and exhibits low-threshold properties with voltage-dependent recovery kinetics 2. KCND1 is widely expressed in human brain, heart, liver, kidney, thyroid, and pancreas tissues 2. Functionally, Kv4.1 channels contribute to regulation of repetitive firing rates in suprachiasmatic nucleus neurons and may influence circadian rhythms, with auxiliary subunits like KCNIP1 and DPP10 modulating channel kinetics [UniProt summary]. Pathogenic variants in KCND1 cause an X-linked neurodevelopmental disorder characterized by developmental delays, neuropsychiatric symptoms, and epilepsy in affected males, while carrier mothers remain unaffected 1. The variants affect diverse channel properties and occur throughout the protein structure, including cytoplasmic domains and transmembrane segments 1. Additionally, KCND1 variants have been identified as potential contributors to nonobstructive azoospermia 3 and pediatric epilepsy 4, suggesting broader roles in male fertility and seizure disorders.