A study published in Cancer & Metabolism reported that knocking down the circadian rhythm gene PER2 in gastric cancer cells can promote resistance to immune checkpoint inhibitors. The mechanism described involves reprogramming the metabolism of myeloid-derived suppressor cells, blunting the expected immune-mediated anti-tumor effects. The data add to a growing body linking circadian biology to immune responsiveness and suggest circadian-linked tumor microenvironment states may influence outcomes under PD-1/PD-L1-based regimens.