MUCL1 (mucin-like 1) is a breast-tissue-restricted protein with emerging roles in cancer pathogenesis and endothelial dysfunction. In breast cancer, MUCL1 functions as a HER2-regulated proliferation driver 1. HER2 activates MUCL1 expression through the phosphoinositide 3-kinase/Akt pathway, and MUCL1 knockdown induces G1/S phase arrest by modulating cyclin D, p21, and p27 levels 1. Mechanistically, MUCL1 operates through the FAK/JNK signaling pathway to promote cell growth 1. In metastatic breast cancer, the transcription factor NR2F1 regulates MUCL1 expression in a specific epithelial subtype associated with poor prognosis 2. Beyond breast cancer, MUCL1 exhibits broader pathological significance. In sepsis, the UCHL1-MUCL1 axis promotes endothelial inflammation and apoptosis via β-catenin/NF-κB pathway activation 3. In melanoma, MUCL1 shows negative correlation with melanogenesis through threonine-dependent mechanisms and autophagy signaling 4. MUCL1 also serves as a biomarker for tolerogenic dendritic cells in autoimmune disease contexts 5 and is upregulated in ER-negative breast cancer under hypoxic conditions 6. Additionally, MUCL1 is elevated in gastric cancer urine proteomes 7 and identified as a hub gene in smoking-induced airway epithelial changes 8. These findings position MUCL1 as a multifunctional protein with therapeutic potential across multiple disease contexts.