Cytochrome P450s are a large family of proteins involved in oxidation reactions. A cytochrome P450 gene from Arabidopsis thaliana, CYP78A9, was isolated in an activation tagging screen in which the Cauliflower Mosaic Virus 35S enhancer was inserted randomly into the genome of Arabidopsis plants homozygous for the apetala2-1 mutation. One line showed extraordinarily wide, heart-shaped ovaries. When the ap2-1 mutation was crossed out of the genetic background, elongated fruits were seen. Many plants, including Arabidopsis, have fruits that will only develop when ovules are present and fertilized. In Arabidopsis plants overexpressing CYP78A9, the siliques keep growing despite failure of fertilization, as is visible in the inflorescence shown on the cover. The resemblance of the wide fruits of plants overexpressing the cytochrome gene in an ap2-1 background to the fruits of Capsella bursa-pastoris indicates the possibility that evolutionary changes in fruit shape may result from altered expression of genes like APETALA2 and CYP78A9.