The study investigates the mechanism by which cGAS (cyclic GMP-AMP synthase) produces a second messenger that activates STING (STimulator of Interferon-G Production). The authors demonstrate that cGAS generates a novel cyclic dinucleotide, >Gp(2′-5′)Ap(3′-5′)>, which is distinct from previously characterized cyclic dinucleotides. This molecule is activated by human STING and is more potent than its bacterial counterpart, cGAMP(3′-5′). The cGAS-catalyzed reaction involves two steps: first, cGAS synthesizes a linear 2′-5′-linked dinucleotide, and then this intermediate is cyclized to form the active 13-membered ring structure. This discovery adds cGAS to the oligoadenylate synthetase family and suggests evolutionary links between DNA and cyclic dinucleotide sensing pathways. The unique chemical linkage in cGAMP(2′-5′) and the two-step synthesis process could be targeted for developing specific inhibitors to treat autoimmune diseases involving the cGAS-STING axis.The study investigates the mechanism by which cGAS (cyclic GMP-AMP synthase) produces a second messenger that activates STING (STimulator of Interferon-G Production). The authors demonstrate that cGAS generates a novel cyclic dinucleotide, >Gp(2′-5′)Ap(3′-5′)>, which is distinct from previously characterized cyclic dinucleotides. This molecule is activated by human STING and is more potent than its bacterial counterpart, cGAMP(3′-5′). The cGAS-catalyzed reaction involves two steps: first, cGAS synthesizes a linear 2′-5′-linked dinucleotide, and then this intermediate is cyclized to form the active 13-membered ring structure. This discovery adds cGAS to the oligoadenylate synthetase family and suggests evolutionary links between DNA and cyclic dinucleotide sensing pathways. The unique chemical linkage in cGAMP(2′-5′) and the two-step synthesis process could be targeted for developing specific inhibitors to treat autoimmune diseases involving the cGAS-STING axis.