Cyclic guanosine monophosphate–adenosine monophosphate (cyclic GMP-AMP, cGAMP) is the first found in metazoa. In mammalian cells, cGAMP is synthesized by cyclic GMP-AMP synthase (cGAS) from ATP and GTP upon cytosolic DNA stimulation. cGAMP produced by cGAS contains mixed phosphodiester linkages, with one between 2'-OH of GMP and 5'-phosphate of AMP and the other between 3'-OH of AMP and 5'-phosphate of GMP.