KDR and esophageal squamous cell carcinoma: Since the mouse-specific MF-1 and DC101 antibodies exerted no significant inhibitory effect on proliferation of human ESCC cells in vitro (Supplementary Figure S2), the observed anti-tumor activity was unlikely to be a direct effect on human cancer cells but could be mediated by mouse non-tumor cells that expressed VEGFR1 or VEGFR2.