According to the National Cancer Institute a biomarker is “a biological molecule found in blood, other body fluids or tissues that is a sign of a normal or abnormal process or of a condition or disease” (NCI) such as cancer.
Biomarkers typically differentiate an affected patient from a person without the disease. There is tremendous variety of biomarkers which can include:
- proteins (e.g. an enzyme or receptor)
- nucleic acids (e.g. a microRNA or other non-coding RNA)
- and peptides among other categories
A biomarker can also be a collection of alterations such as gene expression, proteomic, and metabolomic signatures.
Biomarkers can be detected in the circulation (whole blood, serum, or plasma) or excretions or secretions (stool, urine, sputum, or nipple discharge) and thus easily assessed non-invasively and serially. Biomarkers can also be tissue-derived and require either biopsy or special imaging for evaluation.