tnnc1bis ectopically expressed in fast-twitch fibers of homozygoussox6mutants. Endogenous tnnc1b expression (A) and tnnc1b:eGFP reporter expression (green) (B) is restricted to slow-twitch fibers of wild-type embryos at 48 hours postfertilization (hpf) and excluded from fast-twitch fibers (B′:red) as shown in merged image (B′′). In sox6 mutants, endogenous tnnc1b(C) and the tnnc1b:eGFP reporter (D) are expressed in both slow and fast-twitch fibers (D′: red) as shown in merged image (D′′). Expression of ryr1a is restricted to slow-twitch fibers in wild type (E) but ectopically expressed in the fast-twitch fibers in sox6 mutant embryos at 30 hpf (I). By contrast, myh7b expression remains restricted to slow-twitch fibers in sox6 mutants (cf. F and J). Slow-twitch specific expression of the endogenous smyhc1(G) and the smyhc1:GFP reporter (H) in wild-type embryos. In sox6 mutants (K,L), expression is largely similarly restricted but ectopically expressed in a few fast fibers (arrows).