CUG-binding protein 1 (CUG-BP1) was the first discovered member of the CUG-BP1 and ETR-3-like factors (CELF) family. All CELF proteins possess the same domain structure comprising 3 RNA recognition motifs (RRMs) and a divergent hinge region of 60–90 residues between RRM2 and RRM3. CUG-BP1 performs various functions in posttranscriptional gene regulation, including alternative splicing, translation, and deadenylation. It has been reported that CUG-BP1 plays a role in the pathogenesis of the trinucleotide expansion disease, namely, myotonic dystrophy (DM1), in that it is associated with (CTG)n repeat expansion in the 3′ UTR of myotonin protein kinase (Mt-PK) gene. The (CUG)n repeat region in Mt-PK mRNA is the binding site for CUG-BP1, and the triplet repeat expansion sequesters CUG-BP1 on mutant Mt-PK mRNAs. Recent studies have demonstrated that CUG-BP1 has stronger affinity toward UG repeats than CUG repeats. Since UG repeats are enriched in short-lived transcripts, CUG-BP1 was suggested to be a mediator of UG-motifs-dependent mRNA decay