In this context, the present study underlines the significance of determining microsatellite instability in metastatic disease, demonstrating that the MSI-H phenotype as a predictor of successful immune checkpoint blockade also occurs in tumour types not typical of MMR deficiency-driven pathogenesis such as colorectal or endometrial cancer. The gene discussed is MRC1; the disease is hyperinsulinemic hypoglycemia, familial, 4.