Immunofluorescence staining of sectioned rat mammary tumors showing that protein expression of endothelial cell replication biomarkers is localized to tumor vasculature. Tumor sections were incubated with antibodies against CD31 (red) to visualize tumor vasculature and antibodies against protein biomarkers (green). Each of the four proteins examined (ANGPT2, CLU, EDNRB, and PLAU) were localized specifically to CD31-expressing tumor vasculature (overlay in yellow). For details, see Hardwick et al. in this issue.