In both MC38 and CT26 tumors, the majority of CD8 T cells retained within the tumor for at least 24 h (Kaede Red+) expressed high levels of PD-1, in contrast to those newly entering (Kaede Green+), and furthermore, only these Kaede Red+ cells contained a clear PD-1+ TOX+ population (Fig. 5, A–D). This evidence concerns the gene PDCD1 and neoplasm.