Mutants show defects in the lateral larval muscle pattern. In more than 95% of cases muscles with lateral transverse (LT) morphology are absent. Instead, fibres with abnormal orientation appear in the lateral and ventral regions, but they never insert at LT attachment sites. Muscles DT1 and SBM develop with normal morphologies. Segregation of the muscle founder cells appears unaffected in the mutant embryos, but marker analysis suggests that the specification of the LT founders is altered.