cGAS(cyclic GMP-AMP synthase) also known as MB21D1 is a 522 amino acid cytoplasmic nucleotidyltransferase that catalyzes the formation of cyclic GMP-AMP (cGAMP) from ATP and GTP. cGAS plays a crucial role in innate immune response by acting as a sensor for cytosolic DNA, which is often indicative of viral infection. Upon binding to foreign DNA, cyclic GMP-AMP synthase catalyzes the synthesis of cyclic GMP-AMP (cGAMP) from ATP and GTP, a second messenger that activates the stimulator of interferon genes (STING) pathway, leading to the production of type I interferons and other pro-inflammatory cytokines. This process is vital for initiating an effective antiviral response, highlighting the importance of cyclic GMP-AMP synthase in immune surveillance and defense against pathogens.