This study identifies a novel class of RNA-binding proteins, the triplet repeat expansion (EXP) proteins, which specifically bind to expanded (CUG)n RNAs. These proteins are homologous to the Drosophila muscleblind (mbl) proteins, which are essential for muscle and photoreceptor cell differentiation. The EXP proteins are expressed during mammalian myoblast differentiation and accumulate in nuclear foci in cells with expanded DMPK transcripts, suggesting that they are sequestered by these transcripts. This sequestration hypothesis supports the RNA dominance model for DM1 pathogenesis, where the expanded (CUG)n repeats form RNA hairpins that bind EXP proteins, leading to their recruitment away from other transcripts and subsequent disruption of cellular differentiation. The study provides evidence for the RNA dominance model and suggests that the characteristic muscle wasting and weakness in DM1 patients may be due to the recruitment and sequestration of EXP proteins by (CUG)n expansion RNAs during myoblast differentiation.This study identifies a novel class of RNA-binding proteins, the triplet repeat expansion (EXP) proteins, which specifically bind to expanded (CUG)n RNAs. These proteins are homologous to the Drosophila muscleblind (mbl) proteins, which are essential for muscle and photoreceptor cell differentiation. The EXP proteins are expressed during mammalian myoblast differentiation and accumulate in nuclear foci in cells with expanded DMPK transcripts, suggesting that they are sequestered by these transcripts. This sequestration hypothesis supports the RNA dominance model for DM1 pathogenesis, where the expanded (CUG)n repeats form RNA hairpins that bind EXP proteins, leading to their recruitment away from other transcripts and subsequent disruption of cellular differentiation. The study provides evidence for the RNA dominance model and suggests that the characteristic muscle wasting and weakness in DM1 patients may be due to the recruitment and sequestration of EXP proteins by (CUG)n expansion RNAs during myoblast differentiation.