PDCD1 and neoplasm: Defining the chromatin accessibility by ATAC-seq, two exhausted CD8+ T cell subsets from B16-OVA tumor and Lymphocytic choriomeningitis virus (LCMV) CI13 infection displayed the typical epigenetic characteristics, such as specific exhausted enhancers in the gene encoding Programmed cell death protein-1 (PD-1) and changes in chromatin-accessible regions (ChARs) around Tox.