Approximately 2 percent of cases of thyroid dysgenesis have been shown to result from mutations in genes that code for transcription factors important in thyroid gland development, such as TTF-2, PAX-8, and NKX2. The higher incidence in certain racial and ethnic groups, in preterm infants, in twin and multiple births, and in older mothers points toward genetic or perhaps epigenetic factors that have yet to be discovered. The gene discussed is PAX8; the disease is hypothyroidism, congenital, nongoitrous, 2.