RU.521 (RU320521) is a potent and selective cyclic GMP-AMP synthase (cGAS) inhibitor and inhibits cGAS-mediated interferon upregulation. RU.521 suppresses dsDNA-activated reporter activity with an IC50 of 0.7 μM. RU.521 reduces constitutive expression of interferon in macrophages from a mouse model of Aicardi-Goutières syndrome (AGS) [1].