Fig. 3. The effect of Notch and BMP cross-talk is direct and requires cell-cell contact. (A) Quantitative PCR of Hey1 mRNA expression in C2C12 cells exposed to BMP4 and/or cycloheximide (Chx). (B) Quantitative PCR of Hey1 mRNA expression in C2C12 cells cultured at different densities (20, 40 or 80% confluent) and exposed to various concentrations of BMP4 [25 (+) or 50 (++) ng/ml BMP4]. (C) The amount of Dll1 and Serrate 1 mRNA, as measured by quantitative PCR, in C2C12 cells after addition BMP4.