Invited commentary--"molecular epidemiology": new pathway or new travelling companion?

The term "molecular epidemiology" is coming into general usage, but its implications are ambiguous. A simple definition is that it entails the inclusion in epidemiologic research of biologic measurements made at the molecular level--and is thus an extension of the increasing use of biologically based measures in epidemiologic research. "Molecular epidemiologic" measurements typically detect damaged or naturally variant molecular structures, or use immunologically based techniques to detect particular gene-product molecules. (In contrast, the measurement of biochemical concentration, such as of plasma estrogens, does not require information about molecular structure.) Molecular techniques can be used to measure exposure, early biologic response, or host characteristics that influence susceptibility. They may also elucidate mediating biologic events, and may enable differentiation of adverse health outcomes. Molecular biomarkers have been applied particularly in cancer epidemiology, to measure DNA damage, heritable genetic polymorphisms that influence susceptibility, and "cancer family" genes. Infectious disease epidemiologists use molecular measures of genetic strains of microbes, and they and cancer epidemiologists measure viral nucleic acids within host cells. The term "molecular epidemiology" may suggest the existence of a subdiscipline with substantive new research content. Molecular techniques, however, are directed principally at enhancing the measurement of exposure, effect, or susceptibility, and not at formulating new etiologic hypotheses. As techniques of refinement and elaboration, the integration of molecular measures into mainstream epidemiologic research can offer higher resolution answers in relation to disease causation.