We confirmed in ovo the anti-tumor efficacy of programmed cell death protein-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) checkpoint inhibitors based on the Chicken Chorioallantoic Membrane (CAM) assay, revealing the pertinence of the chicken embryo model in its use for IO research. The gene discussed is CD274; the disease is neoplasm.