Mucin 1 (MUC1), a type I transmembrane glycoprotein, typically resides on the proximal luminal or glandular surface of epithelial cells across various tissues and organs, displaying an apical and polar distribution. However, it's scarcely expressed in the gastrointestinal, uterine, and prostate epithelia. During lactation, it's abundantly present in the mammary gland and is overexpressed in over 90% of breast cancers and advanced epithelial ovarian cancers. Aberrant cytoplasmic and membrane localization of MUC-1 is linked to poor patient outcomes, being positive in adenocarcinomas but negative in squamous carcinomas and nonepithelial malignancies.