Human tissue kallikrein 13 (hK13), also known as KLK-L4 (kallikrein-like gene 4), is a member of the human tissue kallikrein family of serine proteases having diverse physiological functions in many tissues. The KLK13 gene resides on chromosome 19q13.3-4 along with other 14 members in a gene cluster and shares a high degree of homology. KLK13 is a trypsin-like, secreted serine protease expressed specifically in the testicular tissue including prostate, salivary gland, breast, and testis. Growing evidence suggests that many kallikreins are implicated in carcinogenesis and may play a role in metastasis. KLK13 may be involved in the pathogenesis and/or progression of breast and ovary cancers, and is regarded as a novel cancer biomarker. In addition, KLK13 interacts and forms complexes with several serum protease inhibitors, such as alpha2-macroglobulin, and its expression is regulated by steroid hormones.