Molecular Genetics of RNA Processing and Behavior 
tion necessary to the running of the oscillator 
rather than contributing to the "building" of the 
oscillator during development. Until the per 
gene is turned on, the animal cannot tell time. 
These experiments also provide some indication 
as to where the circadian oscillator may reside. 
We have also discovered that the per gene prod- 
uct cycles in a circadian fashion: it is present at 
relatively high levels in the middle of the day. 
Moreover, the per mRNA exhibits a similar "be- 
havior"; the cycling of the per protein is due, at 
least in part, to a cycling of its mRNA. A variety of 
experimental approaches indicate that the per 
protein feeds back to influence the cycling of its 
own mRNA. Recent evidence suggests that a sec- 
ond post-transcriptional regulatory event is also 
necessary for cycling of the per protein. We sus- 
pect that this important feedback loop underlies 
the behavioral rhythms that are observed in nor- 
mal and mutant fruit flies. 
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