Mutations in PNPT1 (polyribonucleotide nucleotidyltransferase 1) represent the first described Mendelian mitochondrial cytopathy associated with enhanced type 1 interferon signalling (27, 28), showing notable phenotypic overlap with the first described type I interferonopathy Aicardi-Goutières syndrome (AGS) (29, 30). Here, PNPT1 is linked to Aicardi-Goutieres syndrome.