Deadenylation of eukaryotic mRNA is a mechanism critical for mRNA function by 
influencing mRNA turnover and efficiency of protein synthesis. Here, we review 
poly(A)-specific ribonuclease (PARN), which is one of the biochemically best 
characterized deadenylases. PARN is unique among the currently known eukaryotic 
poly(A) degrading nucleases, being the only deadenylase that has the capacity to 
directly interact during poly(A) hydrolysis with both the m(7)G-cap structure 
and the poly(A) tail of the mRNA. In short, PARN is a divalent metal-ion 
dependent poly(A)-specific, processive and cap-interacting 3'-5' exoribonuclease 
that efficiently degrades poly(A) tails of eukaryotic mRNAs. We discuss in 
detail the mechanisms of its substrate recognition, catalysis, allostery and 
processive mode of action. On the basis of biochemical and structural evidence, 
we present and discuss a working model for PARN action. Models of regulation of 
PARN activity by trans-acting factors are discussed as well as the physiological 
relevance of PARN.