To identify a tag that could efficiently knock down mouse PD-1, we connected various degron tags to the C-terminus of PD-1, including IKZF1/3, Sall4, ZFP91, and SD, and generated Jurkat human T cell lymphoma cell lines that stably expressed the mouse PD-1-degron tag (Figure 1A). Here, PDCD1 is linked to T-cell non-Hodgkin lymphoma.