Epidermal Growth Factor (EGF) is the founding member of the EGF-family of proteins. Members of this protein family have highly similar structural and functional characteristics. Members of the EGF family are characterized by a shared structural motif, the EGFlike domain, which contains three intramolecular disulfide bonds that are formed by six similarly spaced, conserved cysteine residues. EGF is a growth factor that stimulates cell growth, proliferation, and differentiation by binding to its receptor EGFR. To test the effect of EGF on cell proliferation, Balb/3T3 cells were seeded into triplicate wells of 96-well plates and allowed to attach overnight, then the medium was replaced with various concentrations of recombinant human EGF with serum-free standard DMEM. After incubated for 72h, cells were observed by inverted microscope and cell proliferation was measured by Cell Counting Kit-8 (CCK-8). Briefly, 10 µl of CCK-8 solution was added to each well of the plate，then measure the absorbance at 450 nm using a microplate reader after incubating the plate for 1-4 hours at 37 °C. The dose-effect curve of EGF was shown in Figure 1. It was obvious that EGF significantly increased cell viability of Balb/3T3 cells. The EC50 is 0.411 μg/mL.