CD8A and neoplasm: As such, tumor-infiltrating CD8+ T cells have been recently explored as a starting pool for producing therapeutic T cells because of its enrichment in tumor reactivity; however, this effort has been facing the challenge that a significant portion, if not the majority, of tumor-infiltrating CD8+ T cells undergo exhaustion via the expression of inhibitory receptors such as PD-1, due to the education by an inflammatory tumor microenvironment.