We were intrigued to see that the PD-1 ligands PD-L1(CD274 gene) and PD-L2 (PDCD1LG2 gene) mRNA were among top significant genes that were positively correlated with IDO1 mRNA in lung adenocarcinoma tumors (Fig. 5A, B). Here, PDCD1 is linked to lung adenocarcinoma.