The addition of poly(A) tails to eukaryotic nuclear mRNAs promotes their 
stability, export to the cytoplasm and translation. Subsequently, the balance 
between exonucleolytic deadenylation and selective re-establishment of 
translation-competent poly(A) tails by cytoplasmic poly(A) polymerases is 
essential for the appropriate regulation of gene expression from oocytes to 
neurons. In recent years, surprising roles for cytoplasmic poly(A) 
polymerase-related enzymes that add uridylyl, rather than adenylyl, residues to 
RNA 3' ends have also emerged. These terminal uridylyl transferases promote the 
turnover of certain mRNAs but also modify microRNAs, their precursors and other 
small RNAs to modulate their stability or biological functions.