Kallikrein 9 (KLK9) is a member of the human tissue kallikrein-related peptidase family, encoded by the KLK9 gene on chromosome 19q13.41. It is a secreted serine protease synthesized as an inactive zymogen and activated by proteolytic cleavage. KLK9 is expressed in several tissues, including the brain, skin, and reproductive organs, and is thought to participate in extracellular matrix remodeling and epithelial homeostasis. Biochemical studies show that KLK9 is a chymotrypsin-like enzyme with preference for tyrosine at the P1 site, regulated by metal ions and protease inhibitors. Dysregulated KLK9 expression has been linked to cancers and inflammatory conditions, suggesting potential biomarker and therapeutic relevance.