Two mutagenic events are required to make null mutations of polyhomeotic (ph), which suggests that the locus is complex. Amorphic mutations (ph degrees) die in mid-embryogenesis and completely lack ventral thoracic and abdominal epidermal derivatives, whereas single-event mutations lead to transformations similar to those of known dominant gain of function mutants in the Antennapedia and bithorax complexes. After a chromosomal walk, the ph gene was localized using deficiencies and ph mutations that result from DNA rearrangements. Hybridization analyses show that there are two large, duplicated sequences in the ph region, and DNA lesions affecting either one of these repeats alter the function of the ph locus. We propose a model that may account for this unusual functional organization.