PDCD1 and metastatic colorectal cancer: The first- and second-line therapy for metastatic colorectal cancer (mCRC) consists of fluorouracil-based doublet regimens (5-fluorouracil/leucovorin plus oxaliplatin and 5-fluorouracil/leucovorin plus irinotecan) and biologic-targeted therapies including either endothelial growth factor receptor (cetuximab or panitumumab) targeted treatment or vascular endothelial growth factor (VEGF; bevacizumab), or programmed cell death protein 1 (PD-1) inhibitors [7-8].