Early-onset CRC more often exhibits microsatellite instability-high (MSI-H), high tumor mutational burden (TMB) and concurrent mutation in ARID1A (60%), ATM (60%), FAT1 (60%), FBXW7 (60%), and KMT2B (60%), whereas MSS tumors typically harbor alterations in TP53 (75%), APC (68%), and KRAS (48%). The gene discussed is ARID1A; the disease is colorectal carcinoma.