BMP4 inhibits skeletal muscle specification. Panels I and II: P19[BMP4] and P19[control] cells were aggregated in the presence of 0.8% dimethylsulfoxide (DMSO). P19[control] cells were also aggregated in the absence of DMSO to serve as negative controls. On days 0, 6 and 9 (Panel I) and days 1-5 (Panel II), total RNA was harvested for northern blot analysis and hybridized with the cDNAs indicated on the right. Panel III: P19 cells were transfected with the TOPFlash reporter, aggregated, and treated with the compounds indicated. Cells were harvested 24 hours later for luciferase assays (n = 2). Numbers represent the average +/- standard error of mean and statistics were Student's t-test, *P < 0.05.