大约3％的人类基因组由微卫星或短串联重复区（STR）组成。这些STR经常不稳定，重复单元数量在高频率的扩增（增加）或收缩（减少）。在单个细胞内观察到一些微卫星不稳定性（MSI），并且与某些癌症相关联。第二种MSI形式的特征是基因特异性STR的扩增，这些扩增负责一组40多种人类遗传疾病，称为重复扩增疾病（RED）。虽然错配修复（MMR）途径可以防止全基因组MSI，但新的证据表明，一些MMR因子直接参与了REDS扩增的生成。因此，MMR抑制某些形式的扩展，而某些MMR因子在其他情境下促进扩展。本综述将介绍有关MMR对哺乳动物细胞微卫星扩展的逆向效应的已知信息。

Roughly 3% of the human genome consists of microsatellites or tracts of short tandem repeats (STRs). These STRs are often unstable, undergoing high-frequency expansions (increases) or contractions (decreases) in the number of repeat units. Some microsatellite instability (MSI) is seen at multiple STRs within a single cell and is associated with certain types of cancer. A second form of MSI is characterised by expansion of a single gene-specific STR and such expansions are responsible for a group of 40+ human genetic disorders known as the repeat expansion diseases (REDs). While the mismatch repair (MMR) pathway prevents genome-wide MSI, emerging evidence suggests that some MMR factors are directly involved in generating expansions in the REDs. Thus, MMR suppresses some forms of expansion while some MMR factors promote expansion in other contexts. This review will cover what is known about the paradoxical effect of MMR on microsatellite expansion in mammalian cells.