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We have isolated a Drosophila mutant where the lateral parts of the notum are completely naked, leaving unaffected a median stripe of hairs. This mutation, iroquois (iro), defines a new gene which maps at 69D. We show that, in the presumptive lateral notum of mutant discs, sense organ precursor cells fail to form and the proneural gene scute is not expressed. The expression of a reporter gene inserted near iro suggests that iro itself is massively expressed in this region of the disc. We propose that iro is a prepattern gene essential to activate the expression of scule in the regions of the disc that will form the lateral notum.