Direct binding assays with radiolabeled substrate showed that the association of STING protein (residues 139-379) with [32P]-cGAMP was inhibited in the presence of competing cold cGAMP, c-di-GMP or c-di-AMP, suggesting that the cGAMP binding sites on STING might overlap with those that interact with c-di-GMP and c-di-AMP (Wu J et al.2013). Indeed, mutations of several residues that were shown to participate in the binding of STING to c-di-GMP, including S161Y, Y240S and N242A, also impaired the binding of STING to cGAMP (Yin Q et al. 2012; Wu J et al.2013). Structural study revealed that cGAMP generated by cGAS contains G(2',5')pA and A(3',5')pG phosphodiester linkages, which is distinct from bacterial 3',5' cyclic dinucleotides (Gao P et al. 2013).