Like the isolated high prevalence of Huntington disease in Zulia, Venezuela; maple syrup urine disease among the Mennonites of Pennsylvania, USA; or complete achromatopsia on Pingelap, Micronesia, our study suggests that the frequency of IFNAR1 or IFNAR2 deficiency may be much higher in small, geographically circumscribed populations that have experienced strong genetic drift due to serial founder effects, isolation, or bottlenecks. Here, IFNAR2 is linked to hyperinsulinemic hypoglycemia, familial, 4.