Homeobox genes occupy key positions in the regulatory gene hierarchy responsible for establishing the embryonic body plan in Drosophila, and are thought to play analogous roles in the development of higher vertebrates. The homeobox gene Rpx is expressed during gastrulation in the prospective anterior neural plate and, at later stages, in Rathke's pouch, the primordium of the anterior and intermediate lobes of the pituitary. Extinction of Rpx expression coincides with the differentiation of pituitary-specific cell types. Using lacZ reporter genes, it has been shown that proper spatial and temporal expression in the anterior neural plate can be recapitulated in transgenic mice by as little as 600 bp of upstream sequence and a conserved 180 bp element in the first intron. A region at the 3' end of the gene is required for expression in Rathke's pouch. In addition, an element has been uncovered that directs transgene expression to a region of the hypothalamus and incipient posterior lobe that is in direct contact with Rathke's pouch. In vitro tissue recombination experiments have established that this expression is "induced" by contact with the pouch. It is proposed that this element may be present in other genes that normally respond to signals emanating from the pouch during the determination of the hypothalamic-pituitary axis. Molecular dissection of this element is in progress. The role Rpx plays in pituitary development has been investigated in gain-of-function mouse models. Persistent expression in the pituitary has been achieved in transgenic mice by directing expression of Rpx with pituitary-specific promoters. Transgenic mice bearing alpha-glycoprotein subunit promoter-Rpx fusion genes express Rpx in the pituitary after the endogenous gene normally turns off. These mice are dwarfed and have underdeveloped reproductive tracts. Their pituitaries are very small and are deficient in gonadotroph, thyrotroph, lactotroph, and somatotroph cell lineages. These results indicate that the Rpx gene product must be downregulated during development for proper differentiation of the pituitary. It has been found that the expression of Rpx is not properly silenced during pituitary development in the Ames (df) dwarf mutant, which is phenotypically similar to the alpha-GSU-Rpx transgenic mice. These data indicate that Rpx mis-expression may be causatively involved in the genesis of certain types of pituitary dysfunction in mouse and man.