Complementary DNA (or cDNA) is artificially synthesised from messenger RNA (or mRNA). It is a single-stranded DNA that constitutes the coding part of the region of the genome that has been transcribed. This is called artificial synthesis because the complementary DNA is obtained through a reverse transcription reaction with a reverse transcriptase that allows the transition from messenger RNA to complementary DNA. Indeed, transcription is a natural process in cells and allows the switch from genomic DNA to messenger RNA.  The cDNA can then be used as a template in a variety of downstream applications for RNA studies, such as gene expression; cDNA synthesis is therefore the first step in many molecular biology protocols.

It is interesting to study complementary DNA rather than genomic DNA because only genes transcribed into messenger RNA are active or expressed. In addition, since mRNA does not contain introns (non-coding regions) and other regulatory sequences, cDNA, unlike genomic DNA, also allows researchers to directly determine the amino acid sequence of the peptide encoded by the gene.