Eukaryotic cells have about 10 billions of proteins molecules of 20000 kinds, and the synthesis of most of them takes place in the cytosol. Each newly synthesized protein has a methyonin at the terminal-NH2. However, a high percentage of them suffer co-translational or post-translational modifications. Probably, these modifications give them the necessary information to be specifically directed to the cellular compartment where each protein is required. In the cytosol, at least 100 proteins are modified by post-translational arginilation of terminal-NH2, as it is determined by 2D electrophoresis. From these proteins, 8 have been identified and all of them have glutamic or aspartic acid at the terminal-NH2. Our study is focused on determining why and when proteins are arginilated, and what happens after this modification has occurred. Until now post-translational arginilation has been pointed by the N rule as a determinant in a protein half-life. We are studying the arginilation substrates focusing on their localization and the reason of their arginilation.