Detection of mutant human genes is rapidly becoming an integral part of clinical practice. Human disease may arise by genetic deletion, insertion, fusion, point mutation, or amplification of unstable sequences. Such changes in structure may occur in germ cells or somatically. Rapid advances in understanding the complex nuclear and mitochondrial genomes necessitates deployment of a variety of methods to identify aberrant genes. These techniques include polymerase chain reaction, Southern transfer, and allele- specific hybridization studies, as well as methods to unmask mismatches between mutant and normal sequences. Development of protein truncation tests has added a vehicle for assessing larger DNA segments for mutations that cause premature translational termination. Linkage analysis remains an important tool where direct assay of disease-causing mutations is not possible. Considerations of confidentiality, informed consent, and insurability are important whenever genetic testing is used. These issues will assume increasing importance as presymptomatic testing for heritable predispositions emerges for common conditions.