PDCD1 and colorectal carcinoma: The initial success of programmed cell death 1 (PD-1) blockade in advanced colorectal cancer (CRC) patients was primarily demonstrated in patients with DNA mismatch repair deficiency, leading to FDA approval in 2017 for the treatment of unresectable or metastatic microsatellite instability-high (MSI-H) solid tumors in both adult and pediatric patients (1, 2).