The organization of the anterior pattern in the Drosophila embryo is mediated by the maternal effect gene bicoid. bcd has been identified in an 8.7‐kb genomic fragment by germ line transformants that completely rescue the mutant phenotype. The major transcript of 2.6 kb includes a homeobox with low homology to previously known homeoboxes, a PRD‐repeat and a M‐repeat. In situ hybridizations reveal that bcd is transcribed in the nurse cells. The mRNA is localized at the anterior tip of oocyte and early embryo until the cellular blastoderm stage. The localization of the transcript requires the function of the maternal effect genes exuperantia and swallow while transcript stability is reduced by functions depending on posterior group genes.

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Two new paired domain genes of Drosophila, Pox meso and Pox neuro, are described. In contrast to the previously isolated paired domain genes, paired and gooseberry, which contain both a paired and a homeo‐domain (PHox genes), Pox meso and Pox neuro possess no homeodomain. Evidence suggesting that the new genes encode tissue‐specific transcriptional factors and belong to the same regulatory cascade as the other paired domain genes includes (i) tissue‐specific expression of Pox meso in the somatic mesoderm and of Pox neuro in the central and peripheral nervous system, (ii) nuclear localization of their proteins, (iii) dependence on prd activity and (iv) presence of the paired domain in genes of known regulatory activity. While no mutant phenotypes of Pox meso and Pox neuro have yet been discovered, a murine gene with a paired domain closely homologous to that of Pox meso has recently been identified with the undulated mutant. Both Pox meso and undulated are expressed in tissues derived from the somatic mesoderm. The five known Drosophila paired domains fall into three classes: (i) the prd,gsb‐class, (ii) the Pox meso, undulated‐class and (iii) the Pox neuro‐class which probably includes the paired domain of the murine gene Pax 2.