One of the most striking results of the human (and mammalian) genomes is the low number of protein-coding genes. To-date, the main molecular mechanism to increase the number of different protein isoforms and functions is alternative splicing. However, a less-known way to increase the number of protein functions is the existence of multifunctional, multitask, or ''moonlighting'', proteins. By and large, moonlighting proteins are experimentally disclosed by serendipity. Proteomics is becoming one of the very active areas of biomedical research, which permits researchers to identify previously unseen connections among proteins and pathways. In principle, protein-protein interaction (PPI) databases should contain information on moonlighting proteins and could provide suggestions to further analysis in order to prove the multifunctionality. As far as we know, nobody has verified whether PPI databases actually disclose moonlighting proteins. In the present work we check whether well-established moonlighting proteins present in PPI databases connect with their known partners and, therefore, a careful inspection of these databases could help to suggest their different functions. The results of our research suggest that PPI databases could be a valuable tool to suggest multifunctionality.Moonlighting proteins alternative functions are mostly related to cellular localization, cell type, oligomeric state and the cellular concentration of ligands, substrates, cofactors and products. [1][2][3][4][5] In any case, moonlighting will complicate the analysis and interpretation of protein networks of interactions, functional genomics, metabolomics, knock-out and iRNA phenotypes, genetic analysis of diseases, drug-target identification, toxicology and so on. Although some findings suggest involvement of a protein in extra functions, i.e., finding them in different cellular locations; in amounts exceeding those required for its catalytic known function, usually moonlighting proteins are experimentally disclosed by serendipity; therefore any alternative method to identify these proteins would be very valuable. In a previous work, the possibility of identifying moonlighting proteins by bioinformatics was explored by our group. 6 In the present work, we check whether the analysis of protein interacting partners of well-established moonlighting proteins can be reliable enough to disclose multifunctionality. Fig. 1 Scheme of the method used in this work.