Most patients in the cohort had a monogenic disorder, with more than 10 patients, from most to least frequent, reported for each of the following: beta‐propeller protein‐associated neurodegeneration (BPAN), cerebrotendinous xanthomatosis, Rett syndrome, POLG, SYNJ1, DNAJC6, tyrosine hydroxylase deficiency, Dravet syndrome (SCN1A), neurofibromatosis type I, phosphoglycerate kinase deficiency, and spastic paraplegia type 11. The gene discussed is POLG; the disease is hyperinsulinemic hypoglycemia, familial, 4.