PDCD1 and colorectal cancer: At present, therapies that focus on immune-checkpoint markers, such as programmed cell death 1 (PD-1), programmed cell death ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), are being employed in clinical research to deal with many types of cancers, including non-small cell lung cancer (NSCLC), colorectal cancer (CRC), pancreatic cancer, low-grade serous ovarian carcinoma (LGSOC), and endometrial cancer (EC) [137] (Figure 6).