Imprecise excision of the progenitor insertion, resulting in a deletion of approximately 2.7kb that removes the first exon, but leaves the second exon (which contains the ATG start site) intact.
The myofibrils of the indirect flight muscles of mutant flies are larger than those of wild-type controls when examined ultrastructurally. The mutant myofibrils contain a larger number of thick filaments than control myofibrils.
