Figure 1.

Id2, Id3, and other putative Pax7 target genes are expressed in quiescent satellite cells. (A) CSM4B cells were isolated from the skeletal muscle of 2-mo-old mice. RT-PCR analysis of gene expression in either freshly isolated CSM4B cells or in cells cultured for designated days in vitro is displayed. (B–E) Id3 protein is apparent in satellite cells. Myofibers (and associated satellite cells) were isolated and immunostained with anti-Pax7 and anti-Id3. Pax7⁺, Id3⁺ satellite cell nucleus is indicated by the arrow. Although Id3 staining in the nucleus is specific (and confined to the Pax7⁺ nucleus; arrow), apparent Id3 staining in the myotube is due to autofluorescence.