Figure 2. PDF-like immunoreactivity in fractions separated by HPLC from extracts of 200 whole heads of adult wildtype (OrR) and pdf null mutant (pdf ⁰¹) Drosophila melanogaster. Peak 1 corresponds to M¹⁵ oxidized Drm-PDF, peak 2 to Drm-PDF, and peak 3 to an unknown substance. Unlike peak 3, peaks 1 and 2 were not present in pdf ⁰¹ flies. Thus it is unlikely that the immunoreactivity in peak 3 represents a pdf precursor resulting from propeptide cleavage.