Effect of serum on the assay. HepG2BRA (A) and C2C12BRA (B) cells were incubated with DMEM containing the indicated concentration of rBMP-4 with 0.1% BSA or 1%, 2%, 5% and 10% FBS. Luciferase activity was measured after 15 h of incubation. Background luciferase level in the control sample (no BMP) was subtracted from each of the experimental values. The results are expressed as relative luciferase units (RLU). Each point represents the mean ± SEM of triplicate wells from one representative experiment. Similar results were obtained in two separate experiments.