For example, trends in biomarkers such as carcinoembryonic antigen (CEA), carbohydrate antigen (CA) 19-9, prostate-specific antigen (PSA) and carbohydrate antigen (CA)125 can be monitored serially over time for patents with colorectal cancer, pancreatic cancer, prostate cancer, and ovarian cancer, respectively, as a surrogate for changes in amount of tumor present [9]. Here, KLK3 is linked to prostate cancer.