Appearance on hematoxylin and eosin stains:
(1) disorganized proliferation of small tubular glands with a single epithelial cell layer
(2) infiltrative growth pattern with extension into fat
(3) absence of a tubulocentric configuration
(4) typical absence of a myoepithelial cell layer
(5) absence of elastosis in the stroma
Immunohistochemical features:
(1) calponin, p63, and smooth muscle actin negative (lack of myoepithelial cell layer)
(2) S-100 strongly positive
(3) EMA negative
(4) negative for ER, PR and HER2 ("triple negative")
(5) collagen IV stains an intact circumferential basement membrane
