OSTM1 (osteoclastogenesis associated transmembrane protein 1) is a type I transmembrane protein essential for osteoclast and melanocyte maturation and function 1. As the β-subunit of the ClC-7 chloride/proton exchanger, OSTM1 facilitates 2Cl−/H+ exchange across lysosomal membranes, critical for maintaining lysosomal pH gradients and bone resorption 2. Mechanistically, OSTM1 functions bifunctionally in osteoclasts: it negatively regulates preosteoclast fusion by controlling intracellular calcium signaling and Nfatc1 nuclear translocation, while also regulating endolysosomal trafficking and exocytosis of bone-degrading enzymes like cathepsin K and tartrate-resistant acid phosphatase 3. OSTM1 localizes to the endoplasmic reticulum, trans-Golgi network, and endosomes/lysosomes, where it forms a cytosolic scaffolding complex with kinesin 5B for intracellular cargo trafficking 4. Mutations in OSTM1 cause autosomal recessive osteopetrosis type 5 (OPTB5), the most severe form of osteopetrosis, characterized by abnormally increased bone density, bone marrow occlusion, hematopoietic defects, and neurodegeneration leading to early death 5. Loss-of-function mutations impair osteoclast bone-resorbing capacity, whereas gain-of-function CLCN7 variants cause distinct lysosomal pathology without osteopetrosis 2. Recent studies identify OSTM1 as an E3 ubiquitin ligase involved in copper metabolism regulation 6.