Microsatellite instability testing in #colon #cancer has evolved considerably over the past several years. It began as a way to screen for Lynch #Syndrome, a heritable condition, which accounts for about 5% of colon cancers.

MSI subsequently has been shown to be a prognostic factor in colon cancer.

And in its most recent incarnation as a biomarker, it has been shown to be predictive for response to immunotherapy. Pembrolizumab has been approved for patients with microsatellite instability-high tumors.

Interestingly, or perhaps counter-intuitively, patients with microsatellite unstable tumors actually have a better prognosis. Why is this?