In mice, the 3′–5′ exonuclease Trex1 digests retroelement-derived DNA to suppress the autoimmune response (Stetson et al., 2008), Consistent with this, mutations in human Trex1 cause autoimmune diseases like familial chilblain lupus and Aicardi-Goutieres syndrome (Crow et al., 2006). This evidence concerns the gene TREX1 and Aicardi-Goutieres syndrome.