Fig. 2. Msx1 does not promote cellular proliferation, but alters the expression of cell cycle regulators. (A) Proliferation assays were performed using chicken embryonic fibroblasts infected with a control retrovirus (Vector) or one expressing Msx1. Cells were plated at low density in media containing the indicated percentage of fetal bovine serum. Cell number was estimated by measuring the optical density of Napthal blue-black stained cells and is expressed as percent of the control; error bars indicate standard deviation of assays performed in triplicate. Shown are representative data from assays repeated a minimum of five times. (B) Northern blot analyses were performed using mRNA prepared from undifferentiated C2C12 cells that had been infected with a control retrovirus (Vector) or retroviruses expressing Msx1 or Msx1A. Northern blots contained 2.5 µg of mRNA per lane, and were probed with the indicated cDNAs and visualized by autoradiography. Northern blots were performed a minimum of two times using mRNA prepared from independent experiments; representative data are shown. RPL32 is a control for RNA loading.