At this stage there are lots of cDNA molecules in the test tube. The goal is to count how many different types there are and how many of each type are present.

To do this, scientists use E. coli bacteria. Each bacterium contains a small circular piece of DNA called a plasmid. Scientists are able to insert a single cDNA molecule into each plasmid. This solution of E. coli, in which every bacterium has one cDNA molecule per plasmid is called a "cDNA library".