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Microsatellite instability is a well‐recognised phenomenon. Ten to 15% of sporadic colorectal cancers with a high level of MSI form a well defined group with distinct clinicopathological features. The set of tumours with low level of microsatellite instability (MSI‐low), though widely referred to, is not a clearly defined group. The definitions of MSI‐low have varied among groups and between different studies from the same group. Some studies have found associations between the MSI‐L phenotype and molecular features, notably a higher frequency of K‐ras mutations, and, possibly, methylation of methylguanine methyltransferase. Two recent independent studies, however, showed respectively that 68% and 79%, non‐MSI‐H cancers showed some MSI and could therefore be classed nominally as MSI‐L. There was no evidence for a qualitatively discrete MSI‐L group, but quantitative differences in the level of MSI …