MUC5AC and breast cancer: Immunofluorescence stainings of transwell membrane cryosections show alveolar BC differentiation into a stratified epithelial cell layer with KRT5+/KRT17+/p63+basal cells at the basal region and secretory (SCGB1A1+, MUC5AC+, MUC5B+, MMP7+)-, and ciliated (AcTub+) epithelial cells at the apical region (Fig. 4D).