This work was sponsored by the Mayo Clinic Genomics Education Steering Committee.

Individual reprints of this article are not available. The entire Primer on Medical Genomics will be available for purchase from the Pro-ceedings Editorial Office at a later date. AS = Angelman syndrome; FISH = fluorescence in situ hy-bridization; mtDNA = mitochondrial DNA; PKU = phenyl-ketonuria; PWS = Prader-Willi syndrome; UPD = uniparental disomy After the mapping and sequencing of the human genome, medical professionals from essentially all specialties turned their attention to investigating the role genes play in health and disease. Until recently, medical genetics was considered a specialty of minor practical relevance. This view has changed with the development of new diagnostic and therapeutic possibilities. It is now realized that genetic disease represents an important part of medical practice