What are biomarkers?

PURPOSE OF REVIEW: This article provides working definitions and a conceptual framework to understand the roles of biomarkers in clinical research. RECENT FINDINGS: 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. SUMMARY: 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.