SSRs

Simple sequence repeats (SSRs) often serve to modify genes with which they are associated. The influence of SSRs on gene regulation, transcription and protein function typically depends on the number of repeats, while mutations that add or subtract repeat units are both frequent and reversible. SSRs thus provide a prolific source of quantitative and qualitative variation.

References

 Kashi Y, King DG. Simple sequence repeats as advantageous mutators in evolution. Trends Genet. 2006 May;22(5):253-9. PMID: #16567018#