Early diagnosis of cancer and early detection of relapse following surgery are 
critical for the effective treatment of the disease and for a positive clinical 
outcome. Identification of novel diagnostic, prognostic and predictive 
biomarkers will contribute utmost to clinical decision-making. The human tissue 
kallikrein and kallikrein-related peptidases (KLKs), encoded by the largest 
contiguous cluster of protease genes in the human genome, are secreted serine 
proteases with diverse expression patterns and physiological roles. The aberrant 
expression of KLKs in various malignancies as well as their involvement in many 
cancer-related processes, such as cell growth regulation, angiogenesis, 
invasion, and metastasis, has prompted scientists to investigate their potential 
as cancer biomarkers. Expression of distinct KLKs is associated with 
clinicopathological parameters of cancer patients. Moreover, several KLKs 
possess significant favorable or unfavorable prognostic value in various 
malignancies, with prostate-specific antigen (PSA) being the most widely used 
biomarker in clinical practice, today. KLKs are also considered as very 
promising biomarkers for cancer personalized medicine, especially for prediction 
and monitoring of patients' response to chemotherapy, therefore opening up new 
horizons towards effective patient monitoring post-treatment. This review 
describes the current status of KLKs as tumor biomarkers.