A biomarker can be understood as a molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 that is present (or absent) from a particular cellular type. This facilitates the characterization of a cell type, their identification, and eventually their isolation. Cell sorting techniques are based on cellular biomarkers (for example, Fluorescent-activated cell sorting). A biomarker can be used to identify a cell population, make a diagnostic, measure the progress of disease or the effects of treatment.

One example of cellular biomarker is the protein Oct-4

Oct-4

Oct-4 also known as POU5F1 is a protein that in humans is encoded by the POU5F1 gene. Oct-4 is a homeodomain transcription factor of the POU family. This protein is critically involved in the self-renewal of undifferentiated embryonic stem cells...

 that is found in embryonic stem cell

Embryonic stem cell

Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of the blastocyst, an early-stage embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells...

s, while the Carcinoembryonic antigen

Carcinoembryonic antigen

Carcinoembryonic antigen is a glycoprotein involved in cell adhesion. It is normally produced during fetal development, but the production of CEA stops before birth. Therefore, it is not usually present in the blood of healthy adults, although levels are raised in heavy smokers...

 (CEA) is a tumoral marker used to follow up cancer treatment, and the Prostate Specific Antigen

Prostate specific antigen

Prostate-specific antigen also known as gamma-seminoprotein or kallikrein-3 is a glycoprotein that in humans is encoded by the KLK3 gene. KLK3 is a member of the kallikrein-related peptidase family that are secreted by the epithelial cells of the prostate gland...

(PSA) is used for diagnostic.