SLC12A9 is a lysosomal solute transporter that regulates ammonium and chloride homeostasis, preserving organelle function under metabolic stress 1. The protein localizes to lysosomal membranes and functions as part of a multimeric transport complex; loss-of-function variants cause enlarged lysosomes and impaired cellular ammonia handling. Biallelic loss-of-function mutations in SLC12A9 cause a recessive neurodevelopmental disorder characterized by intellectual disability, skeletal and brain structural abnormalities, congenital heart defects, and hypopigmented hair 2. At the cellular level, patient-derived fibroblasts with pathogenic SLC12A9 variants exhibit enlarged lysosomes that can be corrected by wild-type cDNA, confirming functional causality. Beyond Mendelian disease, SLC12A9 is dysregulated in multiple cancers: upregulation associates with poor prognosis and aggressive phenotypes in uveal melanoma 3, colorectal cancer 4, and glioma 5, where silencing SLC12A9 inhibits cell growth and migration. A rare SLC12A9 variant was also identified in patients with chr7 complex regional pain syndrome type 1 6. These findings establish SLC12A9 as both a disease-causing gene in monogenic lysosomal dysfunction and an emerging biomarker in cancer biology with potential therapeutic relevance.