PDCD1 and gastric cancer: Recent scRNA-seq research on tumor specimens and adjacent normal tissues from nine untreated non-metastatic GC patients displayed that CD8+ T cells exhibited low expression levels of exhaustion markers, including programmed cell death protein-1 (PDCD1), cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), Hepatitis A virus cellular receptor 2 (HAVCR2), lymphocyte-activation gene 3 (LAG-3), and T cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT), which may partly explain the limited benefit of immunotherapy among GC patients [101].