Signal Transduction and the Specification of Cell Fates 
Drosophila, our work may be directly relevant to 
an understanding of development in other organ- 
isms as well. The high levels of homology be- 
tween torso and the gene for platelet-derived 
growth factor (PDGF) and between l( 1 )pole 
hole and Raf- 1 suggest that molecules interacting 
with l( 1 )pole hole might be conserved in the 
Raf-1 pathway of higher vertebrates. 
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