All mammalian mRNAs end with a 200 adenosine residues tail at their 3' ends. Recently several studies have highlighted the role of the poly(A) tail in translation and mRNA stability. A detailed biochemical characterization of the protein components involved in the nuclear polyadenylation have started during last years. Poly(A) polymerase (PAP), necessery for the adenosine addition reaction, has been molecularly cloned from bovine and human. The availability of purified polyadenylation factors and monoclonal antibodies (Mabs) directed against human PAP makes it possible to generate an epitope map of PAP describing the relative location in space of the different epitopes. The epitope map will be related to a linear map which will be obtained by deletion mutagenesis of recombinant PAP followed by Western blot analysis of the various modified PAPs. Comparison of the linear functional maps with the tentative folding map, will allow us to predict a detailed functional/ structural map of PAP. The identification of different epitopes of PAP and their function, will help us to understand the significance of multiple PAPs in mammalian cells.