Project Summary Myotonic dystrophy (DM) is the most common cause of adult-onset muscular dystrophy and the second most common cause of muscular dystrophy overall. Myotonic dystrophy type 1 (DM1) results from an expanded CTG repeat in the 3’ untranslated region of the DMPK gene. The molecular basis for pathogenesis is a toxic gain of function of the RNA transcribed from the mutant allele that contains long tracts of expanded CUG repeats (CUGexp RNA). CUGexp RNA remains in the nucleus bound with proteins to form ribonucleoprotein complexes (RNPs) detected as foci by RNA fluorescence in situ hybridization. CUGexp RNPs include the Muscleblind-Like (MBNL) paralogs, MBNL1 and MBNL2, that are sequestered resulting in their loss of function and a primary cause of pathogenesis. CUGexp RNPs are dynamic nuclear structures that are the cause and therapeutic target of DM1 pathogenesis yet knowledge of the composition of the CUGexp RNP is limited and a full accounting of the mechanisms of CUGexp RNA toxicity in skeletal muscle remains to be established. To gain insight into the molecular and cellular biology of CUGexp RNA and the CUGexp RNP in skeletal muscle, we will use proximity labeling to identify protein and RNA components of the CUGexp RNP complex in addition to MBNL and CUGexp RNA and determine their roles in CUGexp RNA pathogenesis in skeletal muscle. The results will provide the foundation to identify novel therapeutic targets and enhance the efficiency of current approaches targeting the CUGexp RNA component of the RNP.