TREX1 and Aicardi-Goutieres syndrome: AGS is also associated with mutations in 3′-repair exonuclease 1 (TREX1), ribonuclease H2 (RNASEH2A, RNASEH2B, RNASEH2C), RNA-specific adenosine deaminase 1 (ADAR1) and melanoma differentiation-associated protein 5 (MDA5)24–28 all of which are enzymes that play roles in nucleic acid metabolism.