Neuropeptide Y (NPY) belongs to the NPY family of biologically active peptides, which is implicated in the control of feeding and in secretion of gonadotrophin-release hormone. NPY was firstly isolated from brain extracts and found to be one of the most abundant neuropeptides within the brain. Significant release of NPY occurs especially upon stronger sympathetic activation, and recent data suggests that the importance of NPY seems enhanced in stress-related cardiovascular disorders. To test the effect of NPY on cell proliferation, MDA-MB-231 cells were seeded into triplicate wells of 96-well plates and allowed to attach, replaced with various concentrations of recombinant rat NPY. 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 the absorbance at 450 nm was measured using a microplate reader after incubating the plate for 1-4 hours at 37 ℃. Cell viability was assessed by CCK-8 assay after incubation with recombinant rat NPY for 72h. The result was shown in Figure 1. It was obvious that NPY significantly decreased cell viability of MDA-MB-231 cells. The ED50 of recombinant rat NPY is 1.52 μg/ml.