More recently, explorative retrospective studies have revealed a novel marker that might allow for a prognostic subclassification of MSI-H colon cancers in two groups with a different metastatic behaviour: Beta2-Microglobulin (B2M) mutations were detected at a frequency of ∼40% in stage III MSI-H colon cancer lesions, but were completely absent in stage IV MSI-H colon cancers (Kloor et al, 2007). Here, B2M is linked to malignant colon neoplasm.