Classic genetic linkage studies have been used to predict the genetic risk of developing diseases. Until recently, disease-associated polymorphisms could be assessed only by analyzing gene products, e.g., cell surface histocompatibility antigens or blood group substances commonly referred to A, B, H. Advances in molecular biology have now made it possible to measure genetic polymorphisms at the DNA level. This approach utilizes restriction enzyme catalyzed endonucleolytic cleavage and DNA hybridization with gene-specific probes to detect base-pair substitution and fragment length polymorphisms. The potential of this technology is exemlified by the recent identification of individual DNA polymorphisms associated with diabetes mellitus and hemoglobinopathies. A similar molecular approach using specific DNA probes, e.g., oncogenes or ectopic hormones, can be used to potentially detect and characterize DNA polymorphisms in individuals who are oncogenetically predisposed.