In the Drosophila embryo, mutations in the segment polarity gene patched (ptc) cause the replacement of the middle region of each segment by a mirror-image duplication of the remaining structures, including the parasegmental border. This gene, which encodes a transmembrane protein, is initially expressed in a generalized way at blastoderm, but later stops being transcribed in cells expressing the engrailed gene, and even later in cells in the middle of the parasegment. The genes engrailed (en) and wingless (wg) are also segment-polarity genes, and they are expressed in adjacent stripes flanking the parasegment borders in the embryo; in ptc mutants wg expression extends anteriorly and an ectopic stripe of en expression is induced. The suggestion has been made that ptc must be transcribed in a specific subset of cells to prevent en expression anterior to the wg-expressing stripe. Here we report that unrestricted expression of ptc from a heat-shock promoter has no adverse effect on development of Drosophila embryos. The heat-shock construct can also rescue ptc mutants, restoring wg expression to its normal narrow stripe. The ectopic en stripe fails to appear, but the normal one remains unaffected. The results imply that, despite its localized requirement, the restricted expression of ptc does not itself allocate positional information.