In this report, we describe an OSTα deficiency in pediatric patients with fat malabsorption and severe fat-soluble vitamin deficiency, and the impact of treatment with ursodeoxycholic acid (UDCA) on biomarkers of hepatic bile acid synthesis, intestinal fat absorption, and clinical indices, such as growth on full oral nutrition. Here, SLC51A is linked to hyperinsulinemic hypoglycemia, familial, 4.