Of all the common diseases that have a genetic component, IDDM is probably the most tractable to the experimentalist. Large numbers of nuclear multiplex families are available, which can be stored as permanent cell lines; diagnosis is relatively unambiguous; and a mouse strain, the NOD, spontaneously develops autoimmune IDDM similar to the human disorder. In addition, the resolution and accessibility of the human genome map has been revolutionized by the discovery and widespread application of the PCR, particularly the amplification of short, tandemly repeated segments of DNA called microsatellites, which display high levels of allelic polymorphism. With these reagents, the stage is set for dissection of the genetic factors that control the pathophysiology of IDDM.