PECAM1 and cancer: To elucidate the putative niche of highly tumorigenic CD44+ LS1034 cells, we extracted four first generation xenografts en bloc and prepared median frozen section for immunofluorescent detection of CD44+ cancer cells relative to (i) the CD31-positive endothelial cell vascular network and (ii) the pimonidazole hypoxic areas (pO2≤10 mmHg).