USP5 and neoplasm: Although this study has provided enough evidence from biochemical assays and several mouse tumor models to show that USP5 regulates anti-tumor immunity largely through controlling PD-1 protein abundance in CD8+ T cells and affecting the cytotoxic activity of CD8+ T cells, it warrants further in-depth studies to cross the Pd-1 KO mice with Usp5 cKO mice to investigate whether the phenotypes of double KO mice are similar with the phenotypes observed in single KO mice.