One of the best-studied examples is the association of loss-of-function mutations in the gene TREX1 with autoimmune diseases, including Aicardi–Goutières syndrome (ASG)18–20 and systemic lupus erythematosus.21,22TREX1 encodes the enzyme 3′ repair exonuclease 1 (TREX1), a 3′-5′ DNA exonuclease, which degrades cytosolic dsDNA and ssDNA.23–25 It has been proven difficult to directly assess the concentration of cytosolic DNA in both wild-type and TREX1-deficient cells. Here, TREX1 is linked to autoimmune disease.