MBNL1 is an RNA-binding protein that functions as a tissue-specific regulator of pre-mRNA alternative splicing, acting as either an activator or repressor depending on target context 1. It binds to consensus sequences containing YGCU(U/G)Y motifs and shows particular affinity for unpaired uracil residues in stem-loop structures, while tolerating C/G pairing in UGC/GCU-containing motifs 1. In muscle tissue, MBNL1 inhibits cardiac troponin-T (TNNT2) exon inclusion while promoting insulin receptor (IR) exon inclusion, antagonizing CELF protein splicing activity patterns. Beyond splicing, MBNL1 influences circRNA biogenesis by competing with linear splicing through binding sites in its own circularized exon 2. Clinically, MBNL1 dysfunction is central to myotonic dystrophy pathogenesis, where expanded CUG repeats sequester MBNL1, depleting it from the nucleoplasm and causing aberrant splicing of chloride channel transcripts, leading to myotonia 3. MBNL1 also accumulates in intranuclear inclusions in repeat-expansion diseases like oculopharyngodistal myopathy 4. In cancer, MBNL1 expression is dysregulated across multiple tumor types; downregulation correlates with increased metastatic potential in breast, gastric, and esophageal cancers, while upregulation occurs in cervical and colorectal cancers 5. Recent evidence demonstrates MBNL1 degradation via ubiquitination promotes lenvatinib resistance in hepatocellular carcinoma through FAK pathway remodeling 6. MBNL1 focal deletions also occur in pediatric acute myeloid leukemia 7.