Molecular Genetics of Development in Drosophila 
We find that some cells within a given abdominal 
segment express abd-A protein exclusively or pre- 
dominantly; other cells, Ubx protein; and still 
other cells, Abd-B protein. This kind of "mosaic" 
expression can, in theory, specify an unlimited 
number of segmental identities. We have there- 
fore asked the following question: Is the mosaic 
-expression necessary for correct specification of 
these identities? 
To answer this question, we altered flies geneti- 
cally so that homeotic proteins could be ex- 
pressed at will. When Abd-B protein is expressed 
in all cells, thoracic and abdominal segments ex- 
hibit some characteristics of the eighth abdomi- 
nal segment. When both Ubx and Abd-B proteins 
are expressed simultaneously, thoracic segments 
transform to the first abdominal segment, which 
is specified by Ubx, but abdominal segments re- 
main largely unaltered. Therefore, surprisingly, 
we do not observe completely nonsensical iden- 
tities, as might have been expected from this ab- 
normal situation, suggesting that mosaic expres- 
sion is not required, at least at a gross level, for 
correct specification of segments. 
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