MSI-high / dMMR
Tumors with a faulty DNA mismatch-repair system (dMMR) accumulate a distinctive pattern of genetic errors called microsatellite instability (MSI-H). Testing for this — common in colorectal and some other cancers — matters because MSI-H/dMMR tumors respond to checkpoint-inhibitor immunotherapy meaningfully more often than tumors without this feature, making the test a standard part of the initial workup for colorectal cancer.