Abstract Myotonic dystrophy type 1 (DM1) is the leading cause of adult-onset muscular dystrophy and a member of the large family of over 40+ repeat expansions diseases. In 2011, the Ranum lab discovered that many of these expansion mutations can express repetitive proteins without an AUG initiation codon using a novel process called repeat associated non-AUG (RAN) translation. In this initial study, it was demonstrated that the DM1 antisense CAG expansion transcripts express RAN proteins with the antisense RAN polyGln proteins accumulating in disease relevant tissue of DM1 patient and mouse models. While this initial data suggested that RAN proteins might contribute to DM1, the tools and techniques to detect RAN proteins at the time were not sufficiently sensitive to allow for a thorough examination of the role of RAN proteins in DM1. One important unanswered question is the potential role of RAN proteins in the CNS abnormalities of DM1, which have been shown to significantly impact quality of life. Novel and specific RAN protein immunological detection tools newly developed by the Ranum group shows that frequent the DM1 RAN proteins, polyLeu (sense transcript) and polySer, (antisense transcript) accumulate in neurons and glial cells in grey and white matter regions of DM1 frontal cortex. The Ranum lab has also shown that the FDA-approved type 2 diabetes drug metformin inhibits RAN translation and improved disease phenotypes in C9orf72 ALS/FTD mice (ref), suggesting a similar approach may work in DM1. The central hypothesis of this proposal is that RAN translation contributes to DM1 and that modulating RAN translation will mitigate disease features. To test this hypothesis, we will: (1) determine if RAN protein aggregates accumulate in DM1 brain regions that show neuroinflammation and other types of histopathology; (2) examine sense and antisense RAN proteins accumulate in skeletal muscle and if RAN protein accumulation increases with age and muscle pathology; and (3) assess if DM1 RAN proteins are toxic and if inhibiting RAN translation with metformin will improve phenotypes in DM1 iPSC- derived muscle and brain organoids. Taken together these studies will improve our understanding of the role of RAN translation in DM1 and provide strong mechanistic rationale for the use of metformin as a therapeutic strategy for DM1 patients.