Multi-protein complexes are involved in essentially all cellular processes A protein s function is defined by a combination of its own properties its interacting partners and the stoichiometry of each Depending on binding partners a transcription factor can function as an activator in one instance and a repressor in another The study of protein function or malfunction is best performed in the relevant context While many protein complexes can be reconstituted from individual component proteins after being produced individually many others require co-expression of their native partners in the host cells for proper folding stability and activity Protein co-expression has led to the production of a variety of biological active complexes in sufficient quantities for biochemical biophysical structural studies and high throughput screens This article summarizes examples of such cases and discusses critical considerations in selecting co expression partners and strategies to achieve successful production of protein complexes (C) 2010 Elsevier Inc All rights reserved