AGS can be caused by mutations in one of four intracellular enzymes involved in nucleic acid metabolism, including 3′ repair exonuclease (TREX)1 (Crow et al., 2006a), a 3′ exonuclease with substrate preference for single-stranded DNA (ssDNA); RNase H2 (Crow et al., 2006b), which cleaves RNA:DNA hybrids; adenosine deaminase acting on RNA 1 (ADAR1) (Rice et al., 2012), an RNA editing enzyme; and the deoxynucleotide triphosphohydrolase SAM domain and HD domain 1 (SAMHD1) (Rice et al., 2009). This evidence concerns the gene TREX1 and Aicardi-Goutieres syndrome.