In order to better understand the significance of this low-level expression of PD-1 on these antigen-specific KLRG1hi CD8 T cells, we analyzed the co-expression of CTLA-4 as high levels of PD-1 and CTLA-4 on TIL have been shown to identify exhausted CD8+ T cells in melanoma [25, 26]. The gene discussed is CD8A; the disease is melanoma.