KLK4 and cancer: Based on structural comparisons, the substrate specificity of kallikreins is probably defined by residues in the loops surrounding the mouth of the active site and by charged surface regions that can act as exosites for substrate recognition [19], [20] The expression of KLKs is regulated by steroid hormones, resulting in diverse expression patterns and functions, including roles in tissue remodeling, prohormone processing, neural plasticity, as well as cancer-related processes such as cell growth regulation, angiogenesis, invasion, and metastasis [6], [18], [20].