In clinical samples, ARID1A deficiency in colorectal cancer is associated with a right-sided location, microsatellite instability (MSI), medullary histology, higher tumour mutation burden (TMB), higher PD-L1 expression, tumour-infiltrating immune cells, and a higher percentage of consensus molecular subtype I (CMI-1) of the Colorectal Cancer Subtyping Consortium (CRCSC) classification [33,89,90,91,92,93], suggesting that it may respond favourably to immune checkpoint inhibitor therapy [94]. The gene discussed is ARID1A; the disease is colorectal cancer.