Studier's T7 expression system uses the T7 promoter to "turn on" a gene of interest and the T7 RNA polymerase to transcribe that gene into messenger RNA (mRNA) so that E. coli ribosomes can use the RNA-encoded information to synthesize the desired protein. The system can also be used to make desired mRNAs as, for example, was done to make the COVID-19 mRNA vaccines.