The enzyme-catalyzed processing or structural modifications that RNAs such as mRNAs, rRNAs, and tRNAs must undergo before they are functionally finished products. For example, in eucaryotes a block of poly A containing at least 200 AMP residues is enzymatically attached to the 3′ end ofmRNA in the nucleus of the cell. The mRNAs with the “tail” are then transferred to the cytoplasm and the tail enzymatically removed to form the functional mRNAs. It is believed that the poly A tail aids in the transfer of the complex and/or targets the complex to the cytoplasm.