Abdominal segmentation of the Drosophila embryo requires a hormone receptor-like protein encoded by the gap gene knirps

The body pattern along the anterior-posterior axis of the insect embryo is thought to be established by two organizing centres localized at the ends of the egg1. Genetic analysis of the polarity-organizing centres in Drosophila has identified three distinct classes of maternal effect genes that organize the anterior, pos-terior and terminal pattern elements of the embryo2. The factors provided by these gene classes specify the patterns of expression of the segmentation genes at defined positions along the longi-tudinal axis of the embryo3,4. The system responsible for organizing the posterior segment pattern is a group of at least seven maternal genes2 and the zygotic gap gene knirps (kni). Their mutant phenotype has adjacent segments in the abdominal region of the embryo deleted2. Genetic analysis and cytoplasmic transplantation experiments suggested that these maternal genes are required to generate a 'posterior activity' that is thought to activate the expression of kni (reviewed in ref. 2). The molecular nature of the members of the posterior group is still unknown. Here we report the molecular characterization of the kni gene that codes for a member of the steroid/thyroid receptor superfamily of proteins which in vertebrates act as ligand-dependent DNA-binding tran-scription regulators.