Given that the recommended ages for the first and second doses of the measles–mumps–rubella (MMR) vaccine are 8 months and 18 months, respectively, and that the mean age at disease onset in children with KHE in this cohort was 5.2 ± 11.0 months, the MMR vaccine deficiency rate at the time of diagnosis was 94.6%. The gene discussed is MRC1; the disease is hyperinsulinemic hypoglycemia, familial, 4.