Kallikrein-related peptidase 4, also known as PSTS or EMSP1, is a protein which in humans is encoded by the KLK4 gene. This gene is one of the fifteen kallikrein subfamily members located in a cluster on chromosome 19. Kallikreins are a subgroup of serine proteases having diverse physiological functions. Growing evidence suggests that many kallikreins are implicated in carcinogenesis and some have potential as novel cancer and other disease biomarkers. In particular, they may serve as biomarkers for both prostate cancer and breast cancer. It has been found that KLK4 is a proliferative factor with effects on gene expression and it may have a role in prostate cancer development and progression.