Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are caused by genomic expansions of CTG or CCTG repeats. When transcribed, these mutations give rise to repeat expansion RNAs that form nuclear inclusions and compromise the function of myonuclei. Here, we have used in situ hybridization and immunofluorescence to compare DM1 and DM2 and search for proteins that associate with the RNA nuclear (ribonuclear) inclusions. Although muscle disease is generally more severe in DM1, the ribonuclear inclusions were 8‐ to 13‐fold more intense in DM2, implying greater amounts of repeat expansion RNA. Expression of repeat expansion RNA in myoblasts has been implicated in the pathogenesis of congenital DM1. However, we found that repeat expansion RNA is also expressed in myoblasts in DM2, a disorder that has not been associated with a congenital phenotype. Of 10 putative CUG binding proteins tested for colocalization with mutant RNA, only proteins in the muscleblind family were recruited into ribonuclear inclusions. Previous studies have shown activation of the protein kinase, PKR, by expanded CUG repeats in vitro. However, breeding experiments utilizing PKR knockout mice indicate that this kinase is not required for disease pathogenesis in a transgenic mouse model of DM1. We conclude that ribonuclear inclusions are a key feature of the muscle pathology in DM and that sequestration of muscleblind proteins may have a direct role in the disease process. Ann Neurol 2003;54:760–768