PDCD1 and cancer: Studies have shown that PD-1 levels are increased in pancreatic cancer, non-small cell lung cancer, nasopharyngeal cancer, chronic lymphocytic leukemia, advanced rectal cancer, and other cancers (Kruger et al. 2017; Bian et al. 2019; Hejleh et al. 2019; Chang et al. 2019; Meyo et al. 2020; Ruan et al. 2019; Tominaga et al. 2019).