Cyclic GMP-AMP synthase (cGAS, cGAMP synthase), belonging to the nucleotidyltransferase family, is a cytosolic DNA sensor that activates a type-I interferon response. It is part of the cGAS-STING DNA sensing pathway. It binds to microbial DNA as well as self DNA that invades the cytoplasm, and catalyzes cGAMP synthesis. cGAMP then functions as a second messenger that binds to and activates the endoplasmic reticulum protein STING to trigger type-I IFNs production. Mice lacking cGAS are more vulnerable to lethal infection by DNA viruses and RNA viruses.[5][6] In addition, cGAS has been shown to be an innate immune sensor of retroviruses including HIV.