Dose response in bone healing induced by BMP4+VEGF-expressing MDSCs. (a–c) Radiography shows complete bone regeneration induced by two different doses of BMP4+VEGF-expressing cells (B4+Ve). No bone formation occurred in the defects implanted with LacZ+VEGF-expressing cells (Lac+Ve). The ratio of BMP4- to VEGF-expressing cells was kept at 5:1. Arrows mark the edge of the defects or the regenerated bone. (d and e) Quantitative analysis shows the correlation between the amount of bone regenerated and the quantity of transduced cells implanted (*P < 0.05, n = 4).