Endothelial cell-specific STAT3 deletion in mice. Frozen sections of brain tissue harvested from endothelial cell-specific STAT3-deficient mice (STAT3^ECKO) or wild-type (WT) mice were immunostained for CD31 (green) and p-STAT3 (red) to identify endothelial cells and STAT3 activity, respectively. Reduced STAT3 activity was observed in endothelial cells from STAT3^ECKO mice relative to WT mice. Nuclei were stained blue. Images were captured using a Zeiss LSM900 confocal microscope at 20× magnification.