Dear Colleagues,

Regardless of our background, be it cancer research, skin biology, protease inhibition or biochemistry, we have all converged on kallikreins. This family of proteolytic enzymes has emerged as fundamental in numerous processes in living organisms, both with physiological and pathological outcomes. In contrast to other protease families, such as cathepsins or caspases, kallikreins are a multifunctional and multimodal group of predominantly “regulatory” rather than “effector” enzymes. KLK involvement in the inflammatory regulation, growth factor processing, and skin shedding control is essential in homeostasis, yet the very same processes may become detrimental when unleashed, leading to the development of numerous pathologies, with cancer being the most notable. The unprecedented diversity and redundance of the kallikrein family poises a significant challenge when it comes to unveiling their roles. In recent developments, however, more details of their roles and functions have emerged. The crosstalk between KLKs has been described in prostate cancer, and targeted activity-based probes are being developed, while enzyme specificity insight is gaining depth and proteomics approaches are allowing for the unbiased identification of novel protein substrates. Not surprisingly, in the last few years, in parallel with the established topics in KLK biology, new areas of research have emerged. Kallikreins were identified as important players in neurodegenerative diseases and in the development of viral infections, possibly including the one ravaging the world as I write these words.

Kallikrein research has brought us all together; hence, I wholeheartedly invite you to participate in this Special Issue of the Journal of Molecular Sciences to share your exciting results and insight in kallikrein research. At the same time, I genuinely hope that new investigators will find this issue interesting and inspiring and identify its potential to help toward further development of our field. We all converge on kallikreins, after all.

Dr. Tomasz Kantyka
Guest Editor

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• kallikrein
• proteolysis
• cancer
• immunity
• tumor
• Alzheimer’s
• substrate specificity
• inhibition
• regulation