A framework for the molecular classification of circulating tumor markers.

New molecular biological technologies, especially polymerase chain reaction (PCR) and mass spectroscopy, have expanded the pool of molecular targets for cancer diagnosis, monitoring, and prognosis using plasma or serum as the substrate. In this review, a framework is described following the "life history" of a protein--starting with DNA [endogenous (nuclear or mitochondrial) or exogenous (viral)], followed by RNA [endogenous (cell-based or cell-free) or exogenous (viral)], and culminating in protein (either the native protein or the glycan portion of glycoproteins). Each of these levels provides unique opportunities to achieve specificity for cancer diagnosis.