Recent developments in recombinant DNA techniques have allowed an understanding of the molecular genetics of many diseases, some affecting the gastrointestinal tract and liver. DNA probes which detect sequences within or near disease genes can be selected by direct approaches, if the gene product or primary gene function is known, or by indirect methods when the chromosomal location is known. Such probes have resulted in extensive family studies which can now define risks to family members of developing a genetic disease. The development of the polymerase chain reaction will also be of considerable use in clinical genetics and in the diagnosis of some infectious diseases. The techniques are summarized and examples of their use are given. A glossary of terms is also provided.