P{lacW}mbtP3 is inserted in the 5' untranslated region of the mbt transcription unit, 265 bp upstream of the ATG translation start codon.
rhabdomere & eye photoreceptor cell
mbtP3 hemizygous males have a weak rough eye phenotype: There are mild defects in the arrangement and morphology of rhabdomeres and in some ommatidia lack a single photoreceptor cell.