Representative immunohistochemical detection of proliferating mammary endothelial cells. Panel A: Endothelial cells were identified by staining for von Willebrand factor (VWF). Microvessels containing VWF-positive endothelial cells are evident and examples are indicated by arrows. Panel B: Negative control is depicted in which immune serum was replaced by non-immune isotypic control serum. Panels C, D: Examples of proliferative (Ki67-positive) endothelial cells are indicated by arrows. Magnification bar = 50 microns.