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The Myogenic Regulatory Factor family (MRFs) is a family of transcription factors that plays a key role in the development of skeletal muscle. MRFs include Myo D, myf-5, myogenin, and MRF-4. Myo D and myf-5 are genes expressed during myoblast proliferation, whereas myogenin and MRF-4 are expressed during myotube formation and differentiation. Insulin-Like Growth Factor 1 (IGF-1) controls muscle development both embryonically and during postnatal growth and repair by inducing myoblast proliferation and muscle hypertrophy. We determined the mRNA expression patterns of the MRFs during muscle development of the chicken and the effect of recombinant human IGF-1 (rhIGF-1) on these expression patterns by RT-PCR. Myo D displayed a linear increase in expression in both myoblast proliferation and myotube formation after injection of rhIGF-1. Myf-5 expression was altered after rhIGF-1 treatment, but myogenin and MRF-4 both displayed a linear increase during differentiation and myoblast proliferation, respectively. Myostatin, a recently discovered member of the TGF-β superfamily is a negative regulator of skeletal muscle growth. Myostatin knockout (K/O) mice have a two to three fold increase in skeletal muscle mass when compared to their littermates. Various tissues in K/O and control mice were analyzed by RTPCR for Myo D and myogenin expression. No expression of Myo D or myogenin was observed in the non-skeletal muscle tissues. Myo D mRNA expression was higher in the control pectoralis and gastrocnemius muscles (P < 0.05), whereas myogenin mRNA expression was higher in the K/O mouse for the same tissues (P < 0.05).;Finally, finding an appropriate internal standard to use for RNA quantification is essential. Internal standards should be constant in expression throughout development, cell type, and control and experimental treatments. When compared to β-actin, 18S rRNA did not fluctuate in expression during myogenesis of the developing chicken embryo. 18S rRNA varied the least in expression compared to β-actin after treatment to rhIGF-1. Therefore, 18S rRNA is the preferred internal standard for normalizing RNA during embryogenesis and growth factor treatment.