Figure 1

Normal breast duct and terminal duct lobular unit (TDLU) stained with antibodies to cytokeratin (CK)5 and CK14. Note the coexpression of these proteins that form a heterodimer. In this example the luminal cells are the dominantly stained population in the TDLU, but in the duct the myoepithelial cells are stained. Great variability can be seen both within the same breast and between specimens.