Reverse transcriptase is an enzyme found in retroviruses (HIV is an example) that catalyzes the production of DNA from RNA. This allows RNA viruses to change their RNA into DNA which they then attach to the host cell's chromosomes.

Cells with the desired gene (& resulting protein) are grown.

m-RNA from these cells is extracted.

Reverse transcriptase converts the m-RNA to single strand DNA.

DNA polymerase converts single strand DNA to double strand (essentially a gene without introns).

The genes are introduced to bacteria (like E. coli) or cells lacking the gene.

Example:

Insulin gene inserted into E. coli to mass produce insulin for injection into humans.