Individuals with the p.Ala489Thr interferon-induced helicase C domain-containing protein 1 (IFIH1) mutant demonstrate elevated levels of interferon-stimulated genes compared with controls. An interferon score was calculated from the median fold change in RQ value for a panel of six interferon signature genes measured using quantitative polymerase chain reaction – IFI27 (Hs01086370_m1), IFI44L (Hs00199115_m1), IFIT1 (Hs00356631_g1), ISG15 (Hs00192713_m1), RSAD2 (Hs01057264_m1) and SIGLEC1 (Hs00988063_m1) – normalized to the expression levels of HPRT1 (Hs03929096_g1) and 18S RNA (Hs999999001_s1). All three affected members of the family (red) display a score that is comparable with previously reported cases (black)7 with IFIH1-related disease, and is significantly higher than in control samples (****P < 0.0001). Levels were measured on two separate occasions in one child in the reported family.