A microsatellite expansion disorder, myotonic dystrophy type 1 (DM1) is characterized by expanded CTG trinucleotide repeats in the 3' untranslated region of the DMPK gene. A cellular hallmark of DM1 is the aggregation of CUG repeats in the nucleus of cells as RNA foci, which sequester transcription factors and disrupt developmentally-regulated alternative splicing mechanisms and the functions of CELF and MBNL proteins. DM1 is a multisystemic disease, with the leading causes of mortality arising from skeletal muscle degeneration and cardiac arrhythmias. My project focuses on the developmentally-regulated exon switching in SCN5A RNA transcripts that encode cardiac sodium channel protein subunit 5. In DM1, exons 6A and 6B of SCN5A are misspliced, causing the expression of the embryonic protein isoform in adult tissues, thereby inducing cardiac arrhythmias. My goal is to functionally rescue the cardiac arrhythmias exhibited by one of our lab's DM1 mouse models, with the added potential of learning more about the functional effects of SCN5A exon switching.