After the genetic results suggested the possibility of GPI deficiency, the GPI enzyme activity was re-evaluated after suspending the transfusion for 3 months, and the results showed a decrease in enzyme activity to 33.1 EU/g Hb, significantly lower than the normal (55–72 EU/g Hb) value. The gene discussed is GPI; the disease is hyperinsulinemic hypoglycemia, familial, 4.