Validation of PCR. A) Competitive PCR for ActRI was performed using a constant amount of internal standard (0.5 pg) and increasing amount of native ActRI cDNA. A linear relationship was found when the ratio of native ActRI cDNA versus internal standard was plotted as a function of the amount of native ActRI cDNA. B) β-actin PCR was conducted using different amount of template cDNA. There is a linear relationship between the amount of cDNA used and the amount of PCR product generated.