Downregulation of c-myc RNA levels by transforming growth factor (TGF)-β is not observed and is not required for p15INK4B induction in MCF-7 cells. RNA and total protein were extracted from the same flasks of cells treated for either 6 or 24 h with diluent control or 2 ng/ml TGF-β1. The protein samples were probed with an anti-p15INK4B antibody as in Fig. 4b. The Northern blot re-probed with a glyceraldehyde-3-phosphate dehydrogenase (GAPDH) sequence and the image of the 28S and 18S ribosomal RNAs serve as controls.