SAMHD1 and Aicardi-Goutieres syndrome: AGS is caused by mutations in one of seven genes: the 3′ exonuclease TREX1 (Crow et al, 2006a), the deoxynucleoside triphosphate triphosphohydrolase and putative ribonuclease SAM domain and HD domain 1 (SAMHD1; Rice et al, 2009), the adenosine deaminase acting on RNA 1 (ADAR1; Rice et al, 2012), the dsRNA cytosolic sensor IFN‐induced helicase C domain containing protein 1 (IFIH1; Rice et al, 2014) and the three subunits of the ribonuclease H2 (RNase H2) endonuclease complex (RNASEH2A, RNASEH2B and RNASEH2C; Crow et al, 2006b; reviewed by Crow and Manel, 2015).