Review of circulating tumor markers: from enzyme, carcinoembryonic protein to oncogene and suppressor gene.

The ease and non-invasive nature of the procedure for drawing blood has made it possible to measure circulating tumor markers for cancer screening, diagnosis, follow-up and early detection of recurrence. Even though a tumor-specific marker has not yet been found, the specificity and sensitivity of currently used tumor markers have improved over the last several decades as they have progressed from enzyme, hormone and carcinoembryonic protein to monoclonal antibody-defined epitope and finally, in recent years, to oncogene, suppressor gene and their encoded protein product. Both phenotype and genotype are included in these latest new tumor markers. Unlike earlier tumor markers, they can be identified with specific biological processes regulating cell growth, such as the cell cycle, angiogenesis, apoptosis and cell adhesion. Any elevation of these new tumor markers, therefore, can be used to identify defects in a specific metabolic pathway and facilitate the design of effective drug therapy.