Direct analysis of monogenic disorders by means of DNA sequencing, both cDNA and genomic DNA, combined with functional studies, has enabled the molecular basis of a number of diseases to be determined. The development of DNA diagnostics is of particular importance in clinical medicine, and wellestablished methods are now available for the rapid and reliable detection of specific mutations. In the article are presented some of the techniques used, and their application in a few inherited disorders. Particular attention is focused on Gaucher's disease type III (the subacute neuronopathic or Norrbotten type), where a single mutation has been found to be present in all the Swedish cases investigated. The hereditary background of the disease, the basic molecular defect, and detection of the mutation are discussed.