Conventional histology and immunostaining of two benign adenomyoepitheliomas of the breast (A,C,E,G: biphasic; B,D,F: monophasic). A shows a prominent tubular component, whereas in B a spindle-cell architecture predominates (A, H&E, magnification ×20, D, H&E, magnification ×20). C and D display distribution patterns of basal cytokeratin expression that are tantamount to the proportion of tumour cells (immunostaining for Ck5/6, magnification ×20) E and F show expression of sm α-actin (immunostaining for SMA, magnification ×20) with a distribution similar to normal breast tissue in biphasic benign adenomyoepithelioma. G shows double immunofluorescence staining (Ck5/6 green, Ck8/18 red, 40×) with prominent abluminal swarming of Ck5/6-positive cells. The proliferating glandular epithelium consists of differentiated (red signal) and less differentiated (hybrid signal) glandular cells. H shows double immunofluorescence staining (Ck5 red, SMA green, 63×) of a monophasic, malignant myoepithelial tumour with many cells coexpressing SMA and high molecular weigh Ck5. Note some less differentiated cells that express mainly Ck5 and some better differentiated cells with a predominance of SMA expression (green signal).