Dependence of genes expressed in cartilage tissue on BMP-2 with and without vitamin D3 (VD3). All cultures contained β-glycerophosphate [10 mM] and ascorbic acid [50 μg/ml]. Mean ± SD of independent triplicates was quantified by qPCR and is normalized to GAPDH expression. Controls were set as 1. **P < 0,01; ***P = 0,001. OCN = osteocalcin, BSP = bone sialoprotein, Cbfa1 = Core binding factor alpha, ALP = alkaline phosphatase, OPN = osteopontin, ONT = osteonectin, Col1 = Collagen type I. Concentrations of BMP-2 up to 100 ng/ml do not lead to osteoblast-specific expression levels that are reached with VD3 only. 100 ng/ml BMP-2 plus VD3 given together early during differentiation can significantly up-regulate osteoblast genes. However, when VD3 is administered later (d20) in combination with BMP-2, osteoblast gene expression is almost restored.