PDCD1 and colorectal carcinoma: Classification based on high tumor mutational burden (TMB) and microsatellite instability is a subject of intense interest due to clinical trials demonstrating sustained and durable responses to anti-PD1 therapies, such as pembrolizumab, which has led to its recent approval as a first-line therapy for MSI-H/dMMR CRC and for unresectable and metastatic TMB-high solid tumors [46].