As such, MMR-deficient tumors have been found to be particularly sensitive to immune checkpoint inhibition, so much so that the U.S. Food and Drug Administration (FDA) approved the programmed cell death protein 1 (PD-1) inhibitor, pembrolizumab, for unresectable, MMR-deficient, unresponsive solid tumors; this was notably the very first tumor, age, and site agnostic, biomarker-driver approval [17]. This evidence concerns the gene MRC1 and neoplasm.