Tumor markers are endogenous proteins or metabolites whose amounts or modifications are indicative of tumor state, progression characteristics, and response to therapies. They are present in tumor tissues or body fluids and encompass a wide variety of molecules, including transcription factors, cell surface receptors, and secreted proteins. Effective tumor markers are in great demand since they have the potential to reduce cancer mortality rates by facilitating diagnosis of cancers at early stages and by helping to individualize treatments. During the last decade, improved understanding of carcinogenesis and tumor progression has revealed a large number of potential tumor markers. It is predicted that even more will be discovered in the near future with the application of current technologies such as tissue microarrays, antibody arrays, and mass spectrometry. To apply these discoveries to patient care, vigorous validation and assay development for many tumor markers is currently underway. R&D Systems has a wide range of research reagents for use at every stage of the tumor marker development process. These products include a large collection of recombinant proteins, antibodies, and protein quantification kits (Quantikine ELISA kits and DuoSet ELISA development kits).