The expression of six ISGs (IFI27, IFI44L, IFIT1, ISIG15, RSAD2, SIGLEC1) have been measured and used to calculate an IS in the majority of studies involving patients with IMIDs: these ISGs have the greatest differential expression between healthy individuals and patients with Aicardi–Goutières syndrome (AGS), the first defined type I interferonopathy [8, 9]. The gene discussed is RSAD2; the disease is Aicardi-Goutieres syndrome.