Finding the chromosomal location of human genes that heretofore have been defined solely by phenotypes, in particular clinical phenotypes that are transmitted in Mendelian fashion in families, is an early and often crucial step in the process of identifying the molecular basis of a disease. Recent progress in construction of chromosomal maps of genetically linked DNA markers has made almost the entire human genome accessible to linkage studies in families that are segregating genetic defects. Construction of linkage maps requires a panel of three-generation families for genotyping, a large number of polymorphic markers, and sophisticated computer programs for analysis of genotypic data. After a locus harboring a deleterious mutation has been identified by linkage to a mapped marker, a high-resolution map of the region can be constructed with new markers derived from cosmid libraries, to narrow the search for the gene in question. For example, this strategy has been pursued in the effort to characterize the gene responsible for familial adenomatous polyposis. When a target region has been narrowed to about 1 centiMorgan, corresponding to roughly a million base pairs in physical distance, other techniques of molecular biology can be brought to bear to isolate and clone the actual gene.