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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 egg. Genetic analysis of the polarity-organizing centres in Drosophila has identified three distinct classes of maternal effect genes that organize the anterior, posterior and terminal pattern elements of the embryo. The factors provided by these gene classes specify the patterns of expression of the segmentation genes at defined positions along the longitudinal axis of the embryo. The system responsible for organizing the posterior segment pattern is a group of at least seven maternal genes and the zygotic gap gene knirps (kni). Their mutant phenotype has adjacent segments in the abdominal region of the embryo deleted. 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 transcription regulators.

Krüppel is a member of the 'gap' class of segmentation genes of Drosophila melanogaster, mutations of which cause contiguous groups of segments of the fruitfly embryo to fail to develop. In the case of Krüppel mutant embryos, thoracic and anterior abdominal segments are deleted. The molecular cloning of the Krüppel locus will lead to an understanding of the crucial role that gap genes seem to have in early embryonic development. It has already allowed the identification of a blastoderm-specific Krüppel transcript and the phenotypic rescue of mutant embryos by injected cloned DNA.

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