Eighty percent of families with Aicardi–Goutières syndrome have mutations in one of four nuclease genes – the exonuclease Trex1 [chromosome 3p21 (AGS1)] or the genes for all three components of the ribonuclease H2 enzyme complex (AGS2, 3, and 4). The gene discussed is TREX1; the disease is Aicardi-Goutieres syndrome.