In eukaryotic cells, messenger RNAs are formed by extensive post-transcriptional processing of the primary transcripts, assembled with a large number of proteins and processing factors in ribonucleo-protein complexes. The protein part of these complexes mainly constitutes a class of about 20 major polypeptides called heterogeneous nuclear ribonucleoproteins(hnRNPs). In addition to previous models that hypothesised a mere structural function, a more diversified and dynamic role for these protein is now proposed. They might actively participate in the events of RNA metabolism and other cellular functions.