VASA (VAS), a key protein in establishing the specialized translational activity 
of the Drosophila pole plasm, accumulates at the posterior pole of the 
developing oocyte. We identified a gene, gustavus (gus), that encodes a protein 
that interacts with VAS. A gus mutation blocks posterior localization of VAS, as 
does deletion of a segment of VAS containing the GUS binding site. Like VAS, GUS 
is present in cytoplasmic ribonucleoprotein particles. Heterozygotes for gus or 
a deletion including gus produce embryos with fewer pole cells and posterior 
patterning defects. Therefore, GUS is essential for the posterior localization 
of VAS. However, gus is not required for the posterior localization of oskar 
(osk). Apparent gus orthologs are present in mammalian genomes.