Myotonic Dystrophy type 1 (DM1) is a progressive multisystem disease with large heterogeneity in disease onset, symptom development, progression rates and severity. This poses significant challenges to the design of clinical trials. To overcome some of these challenges, this study aims to identify protein biomarkers in serum that can serve as measures of therapy response. Biomarker discovery was performed in serum samples of DM1 patients in the OPTIMISTIC trial, which is deposited elsewhere. The dataset in this project comprises data generated on the Canadian cohort of DM1 patients which was used as external validation cohort.