MBNL1 and myotonic dystrophy type 1: These findings point to a role for MBNL loss in muscle pattern involvement in DM1, indicate additional mechanisms of pathogenicity beyond MBNL sequestration in DM2, and have important implications for the muscle of choice (such as selecting the TA muscle for histological analysis or the soleus muscle for splicing analysis) when performing analyses in mouse models and evaluating potentially new therapeutic modalities and biomarkers.