PD-1 immune checkpoint blockade (ICB) is revolutionizing cancer therapy, but little is known about the mechanisms governing its expression. In a whole-genome, dual-marker FACS-based CRISPR/Cas9 screen in primary murine CD8 T cells, we found that inactivation of the EMP24/GP25L/p24 protein family, most prominently TMED10, reduced PD-1 cell-surface stability.