PECAM1 and glioblastoma: Figure 1 shows that both U87- and GSLC-derived tumors contained murine EC-like cells as stained by an anti-mouse CD31 antibody (Fig. 1A, left). Both U87- and GSLC-derived tumors also contained human EC-like cells as stained by an anti-human CD31 antibody (Fig. 1A, middle). However, GSCL-derived tumors contained a higher number of human EC-like cells than parental U87-derived tumors. In GSLC-derived tumors, 34% of CD31+ cells were of the human origin, whereas almost all CD31+ cells in the tumors formed by parental U87 GBM cells were from mice (Fig. 1A, right).