The Pax-6 transcription-factor gene, containing a paired domain and a paired-type homeodomain, is conserved in structure and ubiquitously present among Metazoa1á-5. It is required for develop-ment of the central nervous system, and is mutated in human aniri-dia, mouse and rat small eye and Drosophila eyeless6. We identified the Pax-6 gene of the nematode Caenorhabditis elegans in genetic studies of male tail morphology7, 8. C. elegans Pax-6 encodes at least two independent genetic functions. One, like other Pax-6 genes, contains paired and homeodomains; this constitutes the genetic locus vab-3 (ref. 9). The other, described here, is expressed from an internal promoter and contains only the homeo-domain portion; this constitutes the genetic locus mab-18 (ref. 7). The mab-18 form of the gene is expressed in a peripheral sense organ and is necessary for specification of sense-organ identity. Its function in this context could be to regulate the expression of cell recognition and adhesion proteins required for sense-organ assembly.
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