IDO1 and cancer: Since immune checkpoint-associated genes participate in the immunosuppressive mechanism that allows tumor cells to escape anti-tumor immunity (40), we next investigated the correlations between PHF19 expression and immune checkpoint-related genes, including BTLA, CD27, CD274, CD276, CD28, CD40, CD70, CD80, CD86, CTLA4, HAVCR2, HHLA2, ICOS, ICOSLG, IDO1, IDO2, LAG3, PDCD1, TIGIT, TNFRSF9, and TNFSF9 across human cancers from the TCGA cohorts, as shown in Figure 4B.