SLC12A9 is a lysosomal solute cotransporter that functions as a potassium-chloride symporter essential for maintaining lysosomal ion homeostasis 1. The protein localizes to the lysosomal membrane 2 and plays a central role in controlling ammonium and chloride levels within lysosomes, protecting cells from ammonia toxicity—a byproduct of cellular metabolism that accumulates in acidic lysosomes 1. SLC12A9 operates as a subunit of a multimeric transport system requiring additional subunits for full function 3. Loss-of-function variants cause lysosomal dysfunction characterized by organelle enlargement and impaired ammonium handling 12. Clinically, biallelic SLC12A9 mutations cause a syndromic neurodevelopmental disorder featuring intellectual disability, skeletal abnormalities, congenital heart defects, and hypopigmented hair 2. Beyond lysosomal physiology, SLC12A9 overexpression is associated with poor prognosis in uveal melanoma and colorectal cancer 45, suggesting roles in cancer metabolism. A rare SLC12A9 variant was identified in complex regional pain syndrome type 1 patients, implicating genetic predisposition 6. This gene represents a fundamental mechanism of lysosomal physiology particularly relevant in high-ammonia environments such as tumors.