Companion diagnostics: the key to personalized medicine. Foreword.

This special focus issue of Expert Review of Molecular Diagnostics on in vitro companion diagnostics aims to provide the reader with up-to-date knowledge on this fast-evolving area of medical research. Companion diagnostics takes up a central role in the development of targeted drugs and to a large extent, the success of this type of therapy depends on their performance. Companion diagnostic assays have a single patient as a point of reference and they will be decisive for the move toward a more precise and individualized pharmacotherapy. The 'first generation' of companion diagnostic assays relies on single biomarker detection but with our increasing understanding of disease pathophysiology a new generation of assays is under development, which will be based on patient profiling and multiplex platforms.