New research shows that microRNA can silence genes that protect against cancer-causing mutations. The study found that microRNA-155 can inhibit the activity of mismatch repair genes, which normally correct damage that occurs when the wrong bases are paired in DNA. Loss of these genes causes cancer-susceptibility syndromes and can contribute to colorectal, uterine, ovarian, and other cancers. It is the first evidence that deregulation of microRNAs can cause genomic instability, a characteristic of cancer cells.