In line with this, a group of genes that were highly increased by RARα‐KO CTLs were transcription factors, such as Irf4, Jun, Egr1, Prdm1, Fos, and Eomes (Figure 6A), which are important for CTL differentiation.[1, 23] The differential expression of IRF4 was verified at protein level in tumor‐infiltrating CTLs of mice bearing MC38 or B16 tumors (Figure S12B, Supporting Information). This evidence concerns the gene IRF4 and neoplasm.