Large quantities of essentially human insulin is made by bacterial cells in vitro. The Insulin messenger RNA (mRNA) is first extracted from human pancreatic cells, then complementary DNA (cDNA) is made using the enzyme reverse transcriptase. Along with a bacterial regulator gene, the cDNA is incorporated (ligated) into the bacterial plasmid. The transformed plasmid vector is then reinserted into a bacterial cell where it replicates. All the plasmids code for the production of proteins, including the human insulin. Large quantities can be made this way for use by diabetics. The insulin is then extracted for human use.