Kinase-negative mutants of VEGFR3 inhibited the differentiation of ES cell–derived VEGFR2⁺ cells into ECs induced by VEGF-C but not by VEGF-A. (A) VEGFR2⁺ cells transfected with mock and VEGFR3 (WT, G857R, R1041P) cDNAs were cultured in SFO3 serum-free medium in the presence of sVEGF-C(WT). After 2 days of culture, cells were observed by phase-contrast microscopy (top row) or doubly stained with anti–PECAM-1 (purple) and anti-SMA (brown) antibodies (bottom row). Scale bar, 100 μm. (B-C) The numbers of cells treated with sVEGF-C(WT) (B) and VEGF-A (C) were determined by Coulter counter.