CD8A and cancer: TCF-1 is expressed on antigen-specific exhausted progenitor CD8+ T cells in mouse models of cancer and chronic infection,18,35 and TCF-1 has been used as a marker of exhausted progenitor CD8+ T cells in tumor immunology studies.27, 28, 29 However, TCF-1 is also expressed on many other subsets of CD8+ T cells, such as naive and memory cells.54