P{lacW} insertion in the first intron upstream of the start codon.
Mutants die at the pupal/pharate adult stage. Mutant embryos show grossly normal morphology, normal segmentation, normal epidermal denticle belts and normal patterning of the neuromusculature. They show greatly reduced movement compared to wild-type. Mature embryos show striking defects in individual muscle fibre morphology.
Excision of the P{lacW} element can give viable revertants. Based on the lethal stage and antibody staining, sls alleles form the series: slsS000201 (weakest), slsB173, slsC995, slsj1D7, slsB68, slsC872 (strongest).