Fig. 6. Cell proliferation is required for Lbx1- and Pax3-mediated activation of myogenesis but not for Dach2 activation. Somites were dissected from HH10 embryos, separated from surrounding tissues and cultured in collagen gels. Tissues were infected with RCAS(A)–Lbx1 (lanes 1, 4, 7 and 10), RCAS(A)–Pax3 (lanes 2, 5, 8 and 11) and RCAS(A)–hAP (lanes 3, 6, 9 and 12) and cultured for 2 days. One set of tissues was left without additives (lanes 1–3), the other set was treated with mitomycin C (lanes 4–6) or irradiated (lanes 7–9) to inhibit cell proliferation. Another set of tissues was incubated with ENU (lanes 10–12). Four days after treatment, tissue explants were analyzed by RT–PCR. In cultures treated with mitomycin C or subjected to irradiation, expression of Lbx1 (lanes 4 and 7) and Pax3 (lanes 5 and 8) did not result in an up-regulation of MyoD, myogenin and MyHC, while treatment with ENU did not affect activation of myogenic markers (lanes 10 and 11).