Imprecise excision of the P{etau-lacZ} insertion causing deletion of the complete drl coding sequence.
In 20% of hemisegments one or more muscles of the 21-23 group pass over their normal ventral attachment sites and appear to attach far more ventrally beyond muscle 13; the 'bypass' phenotype. An additional 10% of hemisegments have more subtle attachment defects where at least one muscle fails to attach within the ventral cluster.