Cathepsin K (CTSK) is a lysosomal cysteine protease predominantly expressed in osteoclasts and belongs to the peptidase C1 protein family. It plays a crucial role in bone remodeling and resorption by degrading collagen and other matrix proteins. CTSK is the most active mammalian collagenase and can cleave both the helical and telopeptide regions of type I collagen, which is abundant in bone. Besides its role in bone metabolism, CTSK is also found in various cancers, such as breast cancer and melanoma, where it contributes to tumor invasiveness and metastasis. Mutations in the CTSK gene can lead to pycnodysostosis, an autosomal recessive disease characterized by osteosclerosis and short stature.