May act as a 'second-level regulator' of head patterning. Required for establishment of the PS(-1)/PS0 parasegmental border and formation of the intercalary segment. Required for expression of the segment polarity genes hedgehog, engrailed and wingless, and the segment-identity genes CAP and collar in the intercalary segment. Required at the onset of the gastrulation for the correct formation of the mandibular segment. Its expression at the blastoderm stage is restricted to a single stripe of cells corresponding to part of the intercalary and mandibular segment primordia, possibly parasegment O. Isoform COL1 is expressed from 3 hours of embryogenesis, with a peak of accumulation between 8 and 16 hours post-fertilization. Expression persists at very low level in first instar larvae and accumulates again in third instar larvae and pupae. Isoform COL2 is expressed after 8 hours of embryogenesis, peaks in first instar larvae and is present at low levels in third instar larvae and pupae. Belongs to the COE family.