The majority of messenger RNA (mRNA) decay in mammalian cells appears to be the 
work of a series of RNA exoribonucleases. A set of multiple poly(A)-specific 
deadenylases has been identified, some, if not most, of which are likely to play 
a role in the key first step of mRNA turnover--the regulated shortening of the 
poly(A) tail. After deadenylation, the transcript likely gets degraded by either 
a 5'-to-3' or a 3'-to-5' exonucleolytic pathway. Interestingly, multiple 
exonucleases have been identified for both of these pathways that appear to form 
multicomponent complexes with diverse roles in cellular biology. Therefore these 
enzymes appear not only to be important components of the mRNA turnover 
machinery, but also may function in a networked fashion in the 
post-transcriptional control of gene expression.