Introduction In the aggregate, genetically determined disease is a major cause of morbidity and mortality. Two studies of the causes of death of more than 1200 children admitted to hospitals in the United Kingdom identified genetically determined diseases as causing 38% and 42% of total mortality. Approximately 3% of all pregnancies result in the birth of a child who has a significant genetic disease or birth defect that can cause crippling, mental retardation, or early death. A recent survey of more than 1 million consecutive births in British Columbia, Canada, indicated that at least 1 in 20 individuals younger than 25 years of age developed a serious disease that had an important genetic component. One third to one half of pediatric hospital admissions involve a disease that has a genetic component. The chronic nature of many genetic diseases imposes a heavy medical, financial, and emotional burden on affected patients and their families as well as on society at large. Patients who have genetic diseases are hospitalized more frequently and for longer periods. The impact of progress in medical genetics is reflected in the pediatric literature. More than 3200 articles per year in the various pediatric journals are concerned with diseases due totally or in part to a genetic variation.