The interaction between cluster of differentiation 47 (CD47) protein overexpressed on numerous types of cancer cells and signal-regulatory protein α (SIRPα) receptor on myeloid cells to convey the “don’t eat me” signal, protecting cancer cells from macrophage-mediated phagocytosis that bears a superficial resemblance to the suppression of T cell activity by adaptive immune cell checkpoint PD-L1/PD-1 [167, 168]. This evidence concerns the gene PDCD1 and cancer.