Reporter expression driven by a short rost promoter fragment is detected first in stage 10-11 embryos, in a segmentally repeated pattern that resembles the pattern of muscle founder cells in the somatic mesoderm. First and strongest expression is in the dorsal mesoderm, at the site of dorsal muscle development. Weaker expression in the overlaying layer is likely due to fusion competent myoblast cells. Expression is later detected in dorsal, lateral and ventral positions, in what appear to be growing muscle fibers at the time the fusion between fusion competent cells and muscle founder cells occurs. At embryonic stage 15-16, single myotubes can be identified: the segmental border muscle (muscle 8) and two of the three ventral oblique muscles group (muscle 15-17), and some dorsal and dorsal-lateral muscles.