Genetic testing revealed a novel activating GCK variant (c.269A>C; p.Lys90Thr), which was also found in the mother (partial pancreatectomy at age 6 for hypoglycemic seizes), his sister (hyperinsulinemic hypoglycemia discovered upon monitoring for gestational diabetes), and the nephew (neonatal hypoglycemia treated with diazoxide). Here, GCK is linked to hyperinsulinemic hypoglycemia.