During the last decade a new class of DNA markers, the restriction fragment length polymorphisms (RFLPs), has been developed by molecular genetic techniques. Genetic linkage studies using RFLPs have resulted in a large number of chromosome assignments of genes, making possible prenatal diagnosis and presymptomatic testing in many genetic disorders. Even so, of the estimated 100,000 genes that comprise the human genome fewer than 2,000, or 2%, have been mapped. Studies of the molecular basis of some of these mutant genes have brought to light a remarkable multiplicity and diversity of mutations that produce relatively few clinical phenotypes. Many genetic disorders including the thalassemias, familial hypercholesterolemia, Tay-Sachs disease, cystic fibrosis, and congenital adrenal hyperplasia, have been shown to be genetically heterogeneous. It is necessary, therefore, to know the precise mutation in order to make accurate diagnosis and restore proper enzyme or gene function.