TCF7 and infection: In addition, a BrdU incorporation assay at day 25 post infection with Cl13 further indicated that the proliferation of Tcf7−/−-exhausted CD8 T cells (PD1+CD8+) was significantly slower than that in the WT group (Figure 3B), which corresponds with the sharp decrease in the expression level of the cellular proliferation marker Ki67 in Tcf7−/− CD8 T cells (Figure 3B).