Project Summary The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas system has become widely adopted for DNA recognition, enabling applications such as genome editing and recruiting effector proteins to DNA to affect transcription or enable imaging. Recently in Nelles et al, Cell, 2016, we showed that with modifications, catalytically dead Cas9 with a modified oligonucleotide containing the PAM recognition sequence is able to bind specific mRNAs in living mammalian cells and is able to track their movement, opening up the potential for many RNA applications of Cas proteins. In this proposal, we aim at developing a adeno-associated virus (AAV)-based therapeutic strategy for myotonic dystrophy, a microsatellite expansion disease characterized by expanded CTG repeats, using RNA-targeting CRISPR/Cas9 (RCas9). We will perform in vivo safety and efficacy studies of our AAV-based therapy and evaluate the hypothesis that alternative RNA processing biomarkers are reliable for measuring treatment efficacy of RCas9 in myotonic dystrophy. If successful, we will have taken a significant step forward in developing a treatment for a class of microsatellite expansion diseases.!