Variegated Position Effects in Drosophila 
during mitosis indicates that instability is asso- 
ciated with premature separation of sister chro- 
matids; as a result, the sisters might sometimes 
independently attach to the spindle apparatus 
that pulls them apart. Current evidence suggests 
that the defect results from a position effect on 
the centromere, because genetic suppressors of 
position-effect variegation also suppress the insta- 
bility phenotype. To understand further the basis 
for instability, we have initiated physical map- 
ping studies on this centromere. Such studies 
should help identify sequences necessary for 
centromere function in a higher eukaryote. 
Phenomena that depend on the position of a 
sequence in the chromosome or on somatic pair- 
ing of homologues are easily observed in Dro- 
sophila, where powerful tools are available for 
genetic dissection. Related phenomena are 
known to occur in mammals, such as X chromo- 
some inactivation, in which one of the female's X 
chromosomes becomes heterochromatic. The 
many similarities between chromosomes in or- 
ganisms as diverse as flies and mammals lead to 
the expectation that an understanding of position 
effects and centromere function in Drosophila 
will have general implications. 
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