As previously discussed, PD-1 is an immune checkpoint receptor expressed on the surface of T cells. The interaction between PD-1 and its ligands (PD-L1 and PD-L2) inhibits T cell activity, allowing cancer cells to evade immune detection. Colon cancer cells can express PD-L1, which binds to PD-1 on T cells, leading to the inhibition of T cell activity and allowing tumor cells to escape immune surveillance. ICIs targeting PD-1 or PD-L1 block this interaction, reactivating T cells and enhancing their ability to attack cancer cells. While expression levels of PD-L1 alone may not be sufficient as predictive biomarkers, there are nonetheless reports studying levels of PD-L1 expression in the context of colon and other cancer types. Instances of these publications appear to be on the rise over the last few years (Supporting Information S1).