Inhibition of proliferative potential by the transfection of siRNA for APN/CD13 in ES-2 cells in a 4-day modified MTT assay. Analysis of the proliferation rate was performed as described in "MATERIALS AND METHODS". si-CD13-transfected cells showed almost no growth, whereas the si-cont-transfected cells grew similarly to the parental ES-2 cells. Closed circles; si-cont-transfected cells, gray squares; si-CD13-transfected cells. Data are expressed as the mean ± SD of four independent experiments. *; p < 0.01, **; p < 0.001.