Recombinant DNA technology, one of the major controversial areas of biological research in the late 1970s, is now rapidly providing new avenues for diagnosis and treatment. With the early recognition that extensive DNA variation exists in human populations, molecular genetic diagnosis of a variety of common hereditary diseases has become a reality. Recent identification of the location of the gene (or genes) for cystic fibrosis and adult polycystic kidney disease, and characterization of the region of the Duchenne muscular dystrophy gene will lead us towards a better understanding of the basic defects in these diseases. The identification of large multi-generation families with genetic diseases that are useful for identifying gene locations will require the co-operative participation of clinicians, medical geneticists and molecular biologists.