<p>Differentiation of the anterior and posterior poles of the Drosophila embryo requires seven maternally expressed genes including torso-like (tsl) and torso (tor). The tor gene encodes a receptor tyrosine kinase that is expressed throughout the embryo but is activated specifically at the poles. Genetic mosaic analysis has shown that tsl is required during oogenesis in follicle cells at each end of the oocyte. We cloned the tsl locus and showed that it was expressed specifically in follicle cells at the anterior and posterior ends of the oocyte. tsl encodes a novel protein with a putative amino-terminal signal sequence. Ectopic expression of tsl produced embryos with a phenotype similar to that resulting from constitutively active Tor alleles. These results suggest that localized TSL controls the localized activation of TOR.</p>