M5 BL21(DE3) Competent Cell is a commonly used plasmid cloning strain with a gene type of F- lon ompT hsdSB (rB-,mB-) dcm gal rne131(DE3). The expression of the T7 RNA polymerase gene is controlled by the lacUV5 promoter of the l phage DE3 region, which is integrated into the BL21 chromosome. This strain is suitable for the recombinant expression of non-toxic proteins. Using pUC19 plasmid detection, the transformation efficiency can reach 106 cfu/μg DNA.