The VEGF-A isoforms play a crucial role in vascular development， and the VEGF signaling pathway is a clinically validated therapeutic target for several pathological conditions. Alternative mRNA splicing leads to the generation of multiple VEGF-A isoforms， including VEGF165. A recent study reported the presence of another isoform， VEGF-Ax， arising from programmed readthrough translation. Compared to VEGF165， VEGF-Ax has a 22-amino-acid extension in the COOH terminus and has been reported to function as a negative regulator of VEGF signaling in endothelial cells， with potent anti-angiogenic effects. Here， we show that， contrary to the earlier report， VEGF-Ax stimulates endothelial cell mitogenesis， angiogenesis， as well as vascular permeability. Accordingly， VEGF-Ax induces phosphorylation of key tyrosine residues in VEGFR-2. Notably， VEGF-Ax was less potent than VEGF165， consistent with its impaired binding to the VEGF co-receptor neuropilin-1.