PRF1 and neoplasm: CTLs play a central role in fighting viral infections and cancers.[2a] In this regard, a positive impact of RA on T cell responses has been described.[10] In addition, RA induces the expression of the gut‐homing receptors CCR9 and α4β7.[11] Vitamin A metabolites and RA signaling are known to increase the expression of granzyme B and perforin to increase CTL activity in acute conditions.[10, 12] RARα was initially implicated in supporting an acute CTL response to infection,[12b] but the intrinsic function of RARα in chronic conditions, such as tumor microenvironments (TME), remains unknown.