This is a phase Ib study of PDR001 in combination with bevacizumab and mFOLFOX6 as first line therapy in patients with metastatic microsatellite stable (MSS) colorectal cancer. The study will assess primarily the safety and tolerability and then the efficacy of PDR001 in combination with bevacizumab and mFOLFOX6. Particular attention will be paid to the level of activity of study drug combinations in CMS4 patients (retrospective analysis).