This strain provides high levels of protein expression. This strain carries the lambda DE3 lysogen, which expresses T7 RNA polymerase from the lacUV5 promoter by isopropyl-1-thio-ß-D-galactopyranoside (IPTG) induction. The mutated rne gene (rne131) encodes a truncated RNase E enzyme that lacks the ability to degrade mRNA, resulting in an increase in mRNA stability. The transformation efficiency of BL-21 is usually low, so the 6 minute/ heat shock cold plating protocol is recommended if high efficiency is desired.