Common diseases are currently defined by their clinical appearance, with little reference to mechanism. Molecular genetics may provide the tools necessary to define diseases by their mechanisms. This is likely to have profound effects on clinical decisions such as choice of treatment and on our ability to characterise more clearly the course of disease and contributory environmental factors. This information also raises the possibility that new therapeutic interventions can be obtained rationally, based on a clear understanding of pathogenesis. Most of these genetic factors will act as “risk factors” and should be managed ethically and practically, as would other risk factors (in hypertension or hypercholesterolaemia, for example). The rapid advances in human molecular genetics seen over the past five years indicate that within the next decade genetic testing will be used widely for predictive testing in healthy people and for diagnosis and management of patients. Molecular genetics was originally used in medicine to map and identify the major single gene disorders, such as cystic fibrosis1 and polycystic kidney disease.2 The excitement in the field has shifted to the elucidation of the genetic basis of the common diseases. With the help of very large, well characterised family collections, genetic linkages for many of the major causes of morbidity and mortality in Western populations have been identified. The genes and DNA variants responsible for these disorders are now being cloned at an ever increasing pace. Large scale genotyping, increasingly integrated genetic and expressed sequence maps,3 and large scale sequencing programmes4 have all contributed to this remarkable evolution in our understanding of how genes might modify our susceptibility to disease. Considering the current rapid acquisition of genetic information relating to common disease and the dramatic technological developments that continue to fuel the field, it would be surprising if most …