KCNJ9 (also known as Kir3.3 or GIRK3) encodes an inwardly rectifying potassium channel that functions as a G-protein-activated channel 1. The channel exhibits greater potassium influx than efflux, with voltage dependence regulated by extracellular potassium concentration and inward rectification mediated by internal magnesium blockade 1. KCNJ9 cannot generate channel activity independently but forms functional tetrameric complexes with KCNJ3/GIRK1 2. The gene is expressed across multiple tissues including pancreas, adipose tissue, and skeletal muscle 1. KCNJ9 serves as a major effector of mu opioid receptor signaling in the striatum 3, with reduced expression associated with decreased severity of withdrawal from opioids, barbiturates, and ethanol 4. Clinical relevance extends to substance use disorders and neurological conditions; KCNJ9 is differentially expressed in cocaine use disorder 5 and identified as a hub gene in comorbid epilepsy-Alzheimer's disease pathology 6. De novo variants in KCNJ9 have been identified in neonatal seizures, with structural modeling suggesting disrupted protein folding may impair channel assembly 2. These findings position KCNJ9 as a candidate therapeutic target for substance use disorders and seizure-related conditions.