KLK13 (kallikrein-related peptidase 13) is a serine protease encoded on chromosome 19.4 that functions as both a tissue-specific protease and immune effector. Primary Function: KLK13 exhibits serine-type endopeptidase activity 1 and is secreted into extracellular compartments including nasal mucus 2. Mechanism: KLK13 cleaves specific protein substrates through proteolytic activity. Notably, it specifically cleaves SARS-CoV-2 spike proteins at sites distinct from other kallikrein family members, with KLK13 preferentially targeting the S1/S2 site 3, thereby inhibiting viral cell entry and syncytium formation 2. Disease Relevance: In breast cancer, higher KLK13 expression associates with favorable prognosis, with an 55-80% reduction in relapse or death risk in hormone receptor-positive and node-positive subgroups 4. Conversely, increased KLK13 expression associates with higher esophageal cancer risk 5. In testicular cancer, KLK13 is significantly downregulated compared to normal tissue 6. In lung cancer, KLK13 promotes malignant cell invasiveness 7. Clinical Significance: KLK13 serves as an independent prognostic marker in breast cancer 4 and represents a potential therapeutic target for coronavirus infections through intranasal delivery 2.