Simple SummaryAlthough the tumor cell-intrinsic cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway plays a crucial role in activating immune cells in the tumor microenvironment in colorectal cancer (CRC), its impact on the infiltration of immune cells and clinical outcomes in patients with mismatch repair proficient/microsatellite stable (pMMR/MSS) CRC has not been thoroughly investigated. In this study, we examine the expression pattern of cGAS-STING in tumor cells and its effect on the infiltrations of CD8+and CD4+T cells, as well as clinical outcomes including survival and recurrence in patients with pMMR/MSS CRC. Our current findings may offer novel insights and therapeutic strategies for patients with pMMR/MSS CRC.AbstractThe cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway plays a crucial role in activating immune cells in the tumor microenvironment, thereby contributing to a more favorable response to immune checkpoint inhibitors (ICI) in colorectal cancer (CRC). However, the impact of the expression of cGAS-STING in tumor cells on the infiltration of CD8+T cells and clinical outcomes in mismatch repair proficient/microsatellite stable (pMMR/MSS) CRC remains largely unknown. Our findings reveal that 56.8% of all pMMR CRC cases were cGAS-negative/STING-negative expressions (cGAS−/STING−) in tumor cells, whereas only 9.9% of all pMMR CRC showed cGAS-positive/STING-positive expression (cGAS+/STING+) in tumor cells. The frequency of cGAS+/STING+cases was reduced in the advanced stages of pMMR/MSS CRC, and histone methylation might be involved in the down-regulation of STING expression in tumor cells. Since the expression level of cGAS-STING in tumor cells has been associated with the infiltration of CD8+and/or CD4+T cells and the frequency of recurrence in pMMR/MSS CRC, decreased expression of cGAS-STING in tumor cells might lead to poor immune cell infiltration and worse prognosis in most pMMR/MSS CRC patients. Our current findings provide a novel insight for the treatment of patients with pMMR/MSS CRC.