FLAG-tags are commonly used for protein abundance measurements and for identification of proteinprotein interactions in living cells. We have observed that the cholera pathogen Vibrio cholerae encodes a FLAG-antibody-reactive protein and identified this protein as an outer membrane porin, Porin4, which contains a sequence very similar to the 3xFLAG epitope tag. We have demonstrated the binding affinity of the conserved peptide sequence (called Porin 4 tag) in Porin4 against monoclonal anti-FLAG M2 antibody. In addition, we created a porin4 deletion mutant, which can be used for background-less FLAG antibody detection experiments. (C) 2020 Elsevier Inc. All rights reserved.