PDCD1 and colorectal cancer: The frequency of NKG2A (0.2% in colorectal cancer, 3.8% in breast cancer, 0.7% in stomach cancer, and 0% in melanoma) in tumor-infiltrated Tregs was lower than that of PD-1 (8.2% in colorectal cancer, 35.1% in breast cancer, and 7.1% in stomach cancer, 0.14% in melanoma).