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.

[1] cGAMP then functions as a second messenger that binds to and activates the endoplasmic reticulum protein STING to trigger type-I IFNs production.

[5][6] In addition, cGAS has been shown to be an innate immune sensor of retroviruses including HIV.

[7][8] The human gene encoding cGAS is MB21D1 on chromosome 6.