Fig. 5. p27^Xic1 synergizes with MyoD to promote muscle differentiation. (A) Embryos were injected in both cells of two-cell stage embryos with MyoD (100 pg), p27^Xic1 (45 pg), p27^Xic1 NT (15 pg), p27^Xic1 CT (50 pg), p27^Xic1 35-96 (50 pg) or a combination thereof. Ventral marginal zones dissected from injected embryos at stage 10 were allowed to develop until parallel embryos reached stage 19 and analyzed by RT-PCR for expression of muscle actin (MA), myosin heavy chain (MHC) and ornithine decarboxylase (ODC) as an internal control. (B) Quantification of the synergistic upregulation of MA by MyoD and p27^Xic1 mutant overexpression in ventral marginal zones. The relative induction of MA was determined by measuring the incorporated [³²P]dATP and normalizing to ODC. The standard error of the mean was obtained by performing this typical experiment in triplicate.