The molecular markers CD4 and CD8—defining helper T cells and killer T cells, respectively—were once considered mutually exclusive. But “double positive” CD4+-CD8+ T cells have lately been discovered in people in various states of health. In a May study published in the Journal of Experimental Medicine, researchers led by Ludwig MSK’s Jedd Wolchok and Taha Merghoub reported their findings on the biology of such double positive (dp) T cells isolated from murine and human melanoma and lung tumors.