Complementary DNA (cDNA) is single-stranded DNA that is complementary to a certain sequence of messenger RNA. It is usually formed in a laboratory by the action of the enzyme reverse transcriptase on a messenger RNA template. Complementary DNA is a popular tool for molecular hybridization or cloning studies. For example, if scientists are cloning a human gene in a bacterium, the long intervening, noncoding sequences called introns get in the way. Thus the scientists must insert a gene with no introns, so they take mRNA from cells and use the enzyme called reverse transcriptase to make DNA transcripts of this RNA. The resulting DNA molecule— cDNA—has the complete coding sequence the researchers need without the introns.

Not all forms of RNA are unstable. Messenger RNA (mRNA) molecules, however, can vary in their stability, depending on their rate of degradation and synthesis in cells and the amount of a particular protein needed.