A near-term market opportunity for cancer companion diagnostic tests exists in drug selection for cancer therapy. Co-development of molecular diagnostics and targeted therapeutics has already been proven to be a successful strategy in the development of novel anti-cancer drugs. Adoption of biomarker development in clinical research provides great opportunities to identify patient subpopulations with differential drug responses and to uncover the underlying mechanisms. These data could help to explain if clinical trials of new drugs are adequate, and offer the possibility of creating a clear prescription path based on predictive biomarkers. The purpose of this TriMark Publications report is to describe the specific segment of the diagnostics market that develops new technology platforms for evaluating the metabolism of therapeutic agents, or for evaluating which therapeutic regimes are most effective for a particular type of disease. The term companion diagnostic means that the particular diagnostic test under evaluation is specifically linked to a known therapeutic drug. This linkage could be important in the therapeutic application and clinical outcome of a drug (personalized medicine), or an important component of the drug development process. This report focuses on the former linkage, i.e., the use of companion diagnostic tests in personalized medicine.