PDCD1 and cancer: In B16F10/pLKO tumors in which progression was inhibited by anti-PD-1 treatment (Fig. 5B, D), anti-PD-1 treatment decreased the percentage of cancer cells in subcluster “Spp1 + ”, while it did not decrease the percentage of cancer cells in other subclusters (Fig. 6G), suggesting that cancer cells in subcluster “Spp1 + ” are anti-PD-1 responders.