Critical comments for roles of biomarkers in the diagnosis and treatment of cancer.

A biomarker is defined as "a characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacologic/pharmacodynamic responses to a therapeutic intervention". Various assays, including immunohistochemistry, gene constitution such as amplification, mutation, and rearrangement, gene and protein expression analysis such as single gene or protein expression, exhaustive analysis and gene or protein signature and single nucleotide polymorphism have been used to identify biomarkers in recent years. No therapeutic effects have yet been predicted based on the results of such exhaustive gene analysis because of low reproducibility although some correlate with the prognosis of patients. Biomarkers such as HER2 for breast cancer or EGFR mutation for lung cancer and KRAS mutation in colon cancer have contributed to identify a patient population that might show a good and bad treatment response, respectively. On the other hand, other biomarkers such as bcr-abl, c-kit gene mutation and CD20 expression, which are positive for CML, GIST and B cell lymphoma, respectively, have crucial biological significance but have not necessarily been used for practical clinical screening since pathological diagnosis coincide with finding of biomarkers. Hence, much work remains to be done in many areas of biomarker research.