Origami™ host strains are K-12 derivatives that have mutations in both the thioredoxin reductase (trxB) and glutathione reductase (gor) genes, which greatly enhance disulfide bond formation in the cytoplasm. Studies have shown that expression in Origami(DE3) yielded 10-fold more active protein than in another host even though overall expression levels were similar (Prinz 1997). Origami hosts are compatible with ampicillin-resistant plasmids and are ideal for use with pET-32 vectors, since the thioredoxin fusion tag further enhances the formation of disulfide bonds in the cytoplasm. The trxB and gor mutations are selectable on kanamycin and tetracycline, respectively; therefore, these strains cannot be used with the plasmids carrying kanamycin or tetracycline resistance genes. To reduce the possibility of disulfide bond formation between molecules, hosts containing the trxB/gor mutation are recommended only for the expression of proteins that require disulfide bond formation for proper folding. In Origami(DE3)pLacI, the rare tRNA genes are present on the same plasmid that carries the lac repressor gene. DE3 indicates that the host is a lysogen of λDE3, and therefore carries a chromosomal copy of the T7 RNA polymerase gene under control of the lacUV5 promoter. Such strains are suitable for production of protein from target genes cloned in pET vectors by induction with IPTG. Genotype: Δ(ara-leu)7697 ΔlacX74 ΔphoA PvuII phoR araD139 ahpC galE galK rpsL F′[lac⁺ lacI^q pro] (DE3) gor522::Tn10 trxB pLacI (Cam^R, Str^R, Tet^R)