Preliminary Events in Olfaction and a Tissue-Specific snRNP Protein 
tions are called snRNPs (small nuclear ribonu- 
cleoproteins; see the report in this volume by 
Joan Steitz, HHMI, Yale University). As might be 
expected for such a fundamental process as splic- 
ing, the individual components of the snRNPs, 
their snRNAs and proteins, are generally present 
in all tissues. 
Usually when snRNPs splice a given pre-mRNA, 
there is only one possible final product. How- 
ever, this is not always the case, particularly with 
regard to RNA processing in the central nervous 
system (CNS). Imagine a movie with more than 
one potential ending, or even alternative scenes 
in the middle, so that individual viewers could 
make their own choices. Alternative mRNA splic- 
ing is a precise process by which different cells. 
starting with the same primary RNA transcript, 
make related, but distinct, mature mRNAs. At 
least one snRNP protein is apparently present in a 
tissue-specific manner, occurring mainly in brain 
and to a much lesser extent in other tissues such 
as heart, adrenal gland, olfactory epithelium, and 
pigment cells. This protein, called N, has been 
identified and cloned in collaboration with Susan 
Amara (HHMI, Yale University). Although N is 
expressed predominately in the CNS, it is only 
present in neurons and not in the more abundant 
nonneuronal cells, such as glia. Because of the 
restricted tissue and cell expression of N, the ge- 
nomic sequence controlling its expression is of 
interest. Recently this sequence has been ob- 
tained and is now being investigated. 
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