What are biomarkers?

This article provides working definitions and a conceptual framework to understand the roles of biomarkers in clinical research.The definitions of the terms discussed in this article--medical signs, symptoms, biomarkers, surrogate endpoints, clinical endpoints, validation--are still under discussion, as are their relationships to each other, but broad consensus has developed in the past decade and a half about the necessity of distinguishing between, in particular, surrogate and clinical endpoints.This article outlines the major definitions of the key terms in this field and considers select cases in which misunderstandings about the terms led to flawed research conclusions.