Myotonic dystrophy (DM1) is an autosomal dominant neuromuscular disorder caused by a (CTG)n expansion in the 3'-untranslated region of the DMPK gene. The RNA dominance model suggests that the DM1 mutation leads to a gain-of-function at the RNA level, where CUG repeats form RNA hairpins that sequester nuclear factors required for muscle development and maintenance. The study identifies triplet repeat expansion (EXP) RNA-binding proteins as potential sequestered factors. These proteins specifically bind to dsCUG RNAs and their binding is proportional to the size of the repeat expansion. The EXP proteins are homologous to Drosophila muscleblind proteins, which are essential for muscle and photoreceptor cell differentiation. EXP expression is activated during mammalian myoblast differentiation, but the proteins accumulate in nuclear foci in DM1 cells. The study supports the RNA dominance model, proposing that DM1 disease is caused by the aberrant recruitment of EXP proteins to the DMPK (CUG)n expansion. The EXP proteins are novel dsCUG-binding proteins that are homologous to Drosophila muscleblind proteins. They are expressed in various tissues and are involved in myoblast differentiation. The study shows that EXP proteins accumulate in nuclear foci in DM1 cells, suggesting that mutant DMPK transcripts sequester these proteins, leading to cellular dysfunction. The findings support the RNA dominance model for DM1 pathogenesis, indicating that the accumulation of mutant DMPK transcripts leads to the sequestration of EXP proteins and subsequent effects on cellular differentiation. The study also highlights the role of EXP proteins in myoblast differentiation and their potential involvement in DM1 disease. The results suggest that the EXP proteins may be involved in the pathogenesis of DM1 by sequestering and recruiting to the DMPK (CUG)n expansion. The study provides evidence for the RNA dominance model, showing that the expansion of CUG repeats leads to the sequestration of EXP proteins, which are essential for normal gene expression. The findings suggest that the sequestration of EXP proteins by mutant DMPK transcripts contributes to the pathogenesis of DM1.Myotonic dystrophy (DM1) is an autosomal dominant neuromuscular disorder caused by a (CTG)n expansion in the 3'-untranslated region of the DMPK gene. The RNA dominance model suggests that the DM1 mutation leads to a gain-of-function at the RNA level, where CUG repeats form RNA hairpins that sequester nuclear factors required for muscle development and maintenance. The study identifies triplet repeat expansion (EXP) RNA-binding proteins as potential sequestered factors. These proteins specifically bind to dsCUG RNAs and their binding is proportional to the size of the repeat expansion. The EXP proteins are homologous to Drosophila muscleblind proteins, which are essential for muscle and photoreceptor cell differentiation. EXP expression is activated during mammalian myoblast differentiation, but the proteins accumulate in nuclear foci in DM1 cells. The study supports the RNA dominance model, proposing that DM1 disease is caused by the aberrant recruitment of EXP proteins to the DMPK (CUG)n expansion. The EXP proteins are novel dsCUG-binding proteins that are homologous to Drosophila muscleblind proteins. They are expressed in various tissues and are involved in myoblast differentiation. The study shows that EXP proteins accumulate in nuclear foci in DM1 cells, suggesting that mutant DMPK transcripts sequester these proteins, leading to cellular dysfunction. The findings support the RNA dominance model for DM1 pathogenesis, indicating that the accumulation of mutant DMPK transcripts leads to the sequestration of EXP proteins and subsequent effects on cellular differentiation. The study also highlights the role of EXP proteins in myoblast differentiation and their potential involvement in DM1 disease. The results suggest that the EXP proteins may be involved in the pathogenesis of DM1 by sequestering and recruiting to the DMPK (CUG)n expansion. The study provides evidence for the RNA dominance model, showing that the expansion of CUG repeats leads to the sequestration of EXP proteins, which are essential for normal gene expression. The findings suggest that the sequestration of EXP proteins by mutant DMPK transcripts contributes to the pathogenesis of DM1.