The aim of this study was to complement information from the immunohistochemical expression of endogenous markers of hypoxia (HIF-1α, CAIX) and vascularity (CD31) with imaging parameters of hypoxia and vascular function from simultaneous [18F]-FMISO-PET/MRI in treatment-naïve ER + breast cancer. Here, PECAM1 is linked to breast carcinoma.