POLG and Rett syndrome: 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.