Until recently, the CRISPR-Cas9 gene editing technique could only be used to manipulate DNA. In a 2016 study, University of California San Diego School of Medicine researchers repurposed the technique to track RNA in live cells in a method called RNA-targeting Cas9 (RCas9). In a new study, published August 10 in Cell, the team takes RCas9 a step further: they use the technique to correct molecular mistakes that lead to microsatellite repeat expansion diseases, which include myotonic dystrophy types 1 and 2, the most common form of hereditary ALS, and Huntington’s disease.