We designed a single-protein production (SPP) system in living E. coli cells that exploits the unique properties of MazF, a bacterial toxin that is an ssRNA- and ACA-specific endoribonuclease. In effect, MazF functions as an "mRNA interferase," because it efficiently and selectively degrades all cellular mRNAs in vivo, resulting in a precipitous drop in total protein synthesis. Concomitant expression of MazF and a target gene engineered to encode an ACA-less mRNA results in sustained and high-level (up to 90%) target expression in the virtual absence of background cellular protein synthesis. Remarkably, target synthesis continues for at least 4 days, indicating that cells retain transcriptional and translational competence despite their growth arrest. SPP technology works well for E. coli (soluble and membrane), yeast, and human proteins. This expression system enables unparalleled signal to noise ratios that should dramatically simplify structural and functional studies of previously intractable but biologically important proteins.