PECAM1 and neoplasm: RIF-1 tumors were much more vascularized than PDX tumors, showing significantly higher CD31 staining (p=0.0002) and microvessel density (p<0.0001) (Figure 3), suggesting that a well-perfused and well-oxygenated tumor environment can be contributing to the non-response to TH-302 in the RIF-1 tumors.