CD8A and melanoma: Outside the thymus, CD4 and CD8 expression on T cells are usually mutually exclusive and tightly regulated by the transcription factors Runx3 and ThPOK (9, 10).Nevertheless, CD4+CD8+ double positive (DP) T cells have been described in intestinal tissue (11), in the blood and/or tissues of patients with chronic viral infections (human immunodeficiency virus, cytomegalovirus and hepatitis C virus) (12–14) and in some cancers (melanoma, breast, and metastatic colorectal cancer) (15–19).