Expression of the breast cancer stem cell marker Aldh1 in lobular breast cancer (LBC). (a) A lobular carcinoma in situ (LCIS) stained for cytokeratin 18 (green) and aldh1 (red). Note the mutually exclusive expression of the two proteins. (b) An invasive lobular carcinoma (ILC) stained for cytokeratin 18 (green) and aldh1 (red). Aldh1-positive cells were frequently observed at invasive fronts. (c) Coexpression of aldh1 (red) in ILC cells positive for active c-Src (green). The asterisk marks an LCIS. (d) Aldh1 expression (red) in ILC cells with strong c-Src activity (green).