Maintained Pax7 expression delays the onset of myogenin expression. Maintained Pax7 expression delays the onset of myogenin expression. Myofibres were cultured in suspension, exposed to retrovirus and then immunostained between 72 and 96 hours later. Infected satellite cell progeny were readily identified by the presence of eGFP. Most cells infected with pMSCV-IRES-eGFP coexpressed eGFP myogenin (a,c,e). Cells infected with pMSCV-Pax7-IRES-eGFP also coexpressed eGFP (Pax7) and myogenin (arrows in b,d,f), showing that the presence of Pax7 did not prevent differentiation. Importantly though, fewer cells coexpressed eGFP and myogenin after pMSCV-Pax7-IRES-eGFP infection compared with control cultures (g). Values are population means ± s.e.m. from 542 (Pax7 RV) and 619 (control RV) satellite cells from two mice and *P<0.05 denotes significant difference from levels in the control using Student's t-test. Counterstaining with DAPI was used to identify all nuclei. Bar, 30 μm. Peter S. Zammit et al. J Cell Sci 2006;119:1824-1832 © The Company of Biologists Limited 2006