Proenkephalin (PENK) is a protein encoded by the PENK gene. It is initially synthesized as a preproprotein and undergoes proteolytic processing to generate multiple products. As PENK has the function of inhibit the migration cells，we measure the activity of recombinant mouse PENK by the ability of the protein to inhibit the migration of U2OS cells. Thus, chemotaxis?assay used 24-well microchemotaxis system was undertaken to detect the inhibition of chemotactic effect of recombinant mouse PENK on U2OS cell line. Briefly, U2OS cells were seeded into the upper chambers and PENK was added in lower chamber with a polycarbonate filter (8 μm pore size) used to separate the two compartments. After incubation at 37℃ with 5% CO2 for 2h, the filter was removed, then cells in low chamber were observed by inverted microscope at low magnification (×10) and the number of migrated cells were counted randomly (five fields for each filter). Result shows MIP-3α is able to induce migration of U2OS cells. The migrated U2OS cells in low chamber at low magnification (×10) were shown in Figure 1.