In patients with previously treated RAS-mutated microsatellite-stable (MSS) metastatic colorectal cancer (mCRC), a multicenter open-label phase 1b/2 trial was conducted to define the safety and efficacy of the MEK1/MEK2 inhibitor binimetinib in combination with the immune checkpoint inhibitor (ICI) nivolumab (anti–PD-1) or nivolumab and another ICI, ipilimumab (anti-CTLA4). This evidence concerns the gene CTLA4 and metastatic colorectal cancer.