Muscle‐specific activation of the EYFP gene. (A) Schematic drawing of the reporter gene LCMV:ECFP(loxP)(FRT)EYFP containing the CMV promoter driving the expression of the ECFP and EYFP genes. The ECFP gene with its polyadenylation signal is flanked by loxP and FRT sites allowing Cre and FLP recombinase‐mediated deletion of the ECFP gene and thus leading to the expression of the EYFP gene. (B and C) Transgenic Xenopus generated by simultaneously introducing the reporter gene LCMV:ECFP(loxP)(FRT)EYFP and the Cre expression vector CAR‐Cre that drives Cre activity by the muscle‐specific actin promoter (11). (B) Lateral view of a living larvae using filters to detect blue (ECFP, left) or yellow (EYFP, right) fluorescence. (C) Detailed ventral view of another larvae using the same filters. M, myofibrils of the jaws; My, myotomes of the tail; G, gall bladder with autofluorescence. The scale bars are 1 mm.