Distribution of ALDH+ cells in benign breast ductules. Brown staining indicates ALDH immunoreactivity. A, ALDH+ cell populations (arrows) located at luminal, basal, and intermediate ductular levels (levels described in detail elsewhere ). B, Micrograph showing an ALDH+ cell population (arrow) and ALDH+ stromal cells (arrowhead; as previously described ). C, Some ALDH+ cells are locatedin ductular bifurcations (arrow; cell type described previously [4, 15]). All sections were counterstained with hematoxylin; scale bars in all three micrographs represent 50 μm.