Insertion in the first intron. In the mutant allele, transcription is initiated in the region downstream of the w gene carried in the inserted P{SUPor-P} element. This downstream region is derived from sequence that is located 3' to the endogenous w locus. In addition, two introns are spliced out of this region in the mutant transcript. The first intron is located in the region downstream of w and the second intron splices to the next downstream exon of GlcAT-S.