Zerknüllt (German for "crumpled") is a gene in the Antennapedia complex of Drosophila (fruit flies), where it operates very differently from homologous genes in other organisms.
Zerknüllt codes for a homeoprotein that helps shape the fly's tissue during embryogenesis. In Drosophila, zerknüllt's expression is repressed by dorsal protein. This normally limits its expression to roughly the dorsal 40% of the embryo, where the concentration of dorsal protein is low. Zerknüllt's homologs are hox genes, common to a vast range of organisms evolutionarily very distant from Drosophila, even including mammals. Hox genes are activated by genetic regulation mechanisms that target each gene's effect much more narrowly: to just one segment of the developing embryo, corresponding to the gene's position in the chromosome sequence.
Zerknüllt is commonly abbreviated zen.