Cathepsin K (CTSK) is a lysosomal cysteine protease of the papain superfamily that plays a central role in bone homeostasis and extracellular matrix degradation 1. The gene spans approximately 12.1 kb on chromosome 1, consisting of eight exons and seven introns 2. CTSK is abundantly expressed in osteoclasts where it performs specialized functions in bone resorption 2, and is also expressed by immune cells including dendritic cells, macrophages, and T cells, where it modulates inflammation and immunity through NF-κB, TLR, and RANKL/RANK/OPG signaling pathways 1. Beyond bone, CTSK participates in extracellular matrix disassembly in multiple tissues. In the cornea, strong CTSK expression is associated with pathological collagen destruction in keratoconus 3. In skeletal muscle, elevated CTSK expression correlates with muscle injury and necrosis in Duchenne muscular dystrophy 4. Loss-of-function mutations in CTSK cause pycnodysostosis, a rare autosomal recessive skeletal dysplasia characterized by osteosclerosis, distal phalanx osteolysis, and craniofacial dysmorphism 5. Conversely, CTSK overexpression appears in rheumatoid arthritis, periodontitis, atherosclerosis, and castration-resistant prostate cancer, where elevated CTSK correlates with disease activity and tissue destruction 16. Small-molecule inhibitors and monoclonal antibodies targeting CTSK are emerging as therapeutic options for these inflammatory and malignant conditions 1.