New molecular techniques concerned with the isolation and identification of DNA fragments can be used for carrier detection and early prenatal diagnosis either by direct detection of the mutant DNA sequence or by indirect linkage studies employing RFLPs as DNA markers. Gene specific DNA probes are available already for a number of genetic disorders, such as the hemoglobinopathies, hemophilia A and B, alpha 1-antitrypsin deficiency, phenylketonuria, and chronic granulomatous disease. Coinheritance of DNA-polymorphisms can be traced, eg, for Norrie disease, myotonic dystrophy, Duchenne and Becker muscular dystrophies, and Huntington chorea. Several genes have been localized successfully to specific chromosome regions. By "walking" or "jumping" along the chromosome, it is hoped finally to reach further gene loci of interest, to analyze the molecular pathology of single gene disorders, and to find new ways for their prevention.