These include the PD-1.5 (rs2227981), PD-1.6 (rs10204525), and PD-1.9 (rs2227982) polymorphisms which were correlated with lung adenocarcinoma [31], cervical [32], lung [33], gastric [34], and thyroid cancers [35], and esophageal cancer [26]. Here, PDCD1 is linked to esophageal cancer.