Invasive apocrine carcinoma, especially the histiocytoid variant (shown here), can sometimes be difficult to distinguish from granular cell tumor which does rarely arise in the breast. Both entities can have cells with eosinophilic granular cytoplasm in a dense collagenous stroma. Distinguishing features of granular cell tumor include: small hyperchromatic nuclei lacking atypia; positivity for CD68 and S-100 protein; negative for cytokeratin AE1/3. Apocrine carcinoma shows nuclear pleomorphism and prominent nucleoli. It is positive for cytokeratin AE1/3 and negative for CD68. S-100 protein may be focally weakly positive in apocrine carcinoma.