PECAM1 and neoplasm: Ex vivo immunofluorescence staining for CD31 as a marker of endothelial cells and average microvessel density (a-MVD) were used for a better characterization of tumor vascularization: fewer vessels were observed in 4T1 tumors than in 67NR ones (18 ± 3 and 33 ± 8 respectively, p < 0.01, Fig. 4E, F), which may explain the in vivo results of tumor vessel permeability and correlate with the larger necrotic areas (although more limited in number) observed in 4T1 tumors.