In breast tissue, which marker category indicates the presence of an intact myoepithelial layer, helping distinguish in situ from invasive carcinomas?

• A. EGFR
• B. CK5/6
• C. HER2
• D. Myoepithelial

Answer: (D)

The presence of an intact myoepithelial layer around breast ducts is the clue used to distinguish in situ from invasive carcinomas. This layer sits between the ductal epithelium and the basement membrane, acting as a barrier. If the tumor is in situ, the myoepithelial layer remains continuous and can be detected with myoepithelial markers, signaling the lesion is confined. If invasion occurs, that layer is disrupted or absent, and those markers no longer outline the tumor nests.

Common myoepithelial markers include p63 (nuclear staining), calponin, and smooth muscle actin (SMA), which label the myoepithelial cells and reveal the layer’s presence. Other categories listed—EGFR and HER2—are receptors unrelated to labeling the myoepithelium, while CK5/6 marks basal cytokeratins and can be seen in various basal-like contexts but does not specifically indicate an intact myoepithelial barrier.