cis regulatory region. (A) Trunk region of a Tg(BACtnnc1b:eGFP)i293 larva 72 hpf showing reporter gene expression specifically in the slow-twitch muscle. (B) Cross section through the mid-trunk region of a 30 hpf Tg(BACtnnc1b:eGFP)i293 embryo showing reporter gene expression restricted to the superficial layer of muscle fibers: (B’) the same section showing endogenous tnnc1b mRNA assayed by fluorescent in situ hybridization. (B”) Merged images showing that the reporter faithfully recapitulates the expression pattern of the endogenous gene. (C) Schematic representation of deletion derivatives of the BACtnnc1b:eGFP reporter construct (Green box = eGFP); the expression patterns observed in transgenic lines carrying these construct is indicated on the right. (D) Cross section and (E) optical sagittal section of a 72 hpf transgenic larva showing expression of eGFP throughout the myotome driven by the first intron of the zebrafish tnn1cb intron and a mammalian βglobin minimal promoter. (F) Cross section of a 72 hpf transgenic embryo in which expression of eGFP is driven by the first intron of the human slow troponin c gene. The strong signal in the skin in is a staining artefact and was not present in live embryos.