VEGFA (Vascular endothelial growth factor A) is a growth factor and can be cleaved into several isoforms, including VEGF121. It induces endothelial cell proliferation, promotes cell migration, inhibits apoptosis and induces permeabilization of blood vessels. It is accepted that the VEGF121 isoform stimulates the proliferation of vein endothelial cells. Thus, proliferation assay of recombinant human VEGF121 was conducted using ECV-304 cells. Briefly, ECV-304 cells were seeded into triplicate wells of 96-well plates at a density of 2,000 cells/well and allowed to attach overnight, then the medium was replaced with serum-free standard 1640 prior to the addition of various concentrations of VEGF121. After incubated for 48h, cells were observed by inverted microscope and cell proliferation was measured by Cell Counting Kit-8 (CCK-8). Briefly, 10uL of CCK-8 solution was added to each well of the plate, then the absorbance at 450nm was measured using a microplate reader after incubating the plate for 1-4 hours at 37 degree C. Proliferation of ECV-304 cells after incubation with VEGF121 for 48h observed by inverted microscope was shown in Figure 1. Cell viability was assessed by CCK-8 (Cell Counting Kit-8 ) assay after incubation with human recombinant VEGF121 for 48h. The result was shown in Figure 2. It was obvious that VEGF121 significantly increased cell viability of ECV-304 cells.