PDCD1 and colorectal cancer: In addition, Siglec-9 has been found to be upregulated on tumor-infiltrated CD8+ T cells in non-small-cell lung cancer and ovarian and colorectal cancers, and other inhibitory receptors, such as PD-1, are also coexpressed by T cells expressing siglec-9, implying that combination with other immune checkpoint inhibitors could be used for coinhibition in immunotherapy (122).