Complementary DNA (cDNA) is a doublestranded DNA version of an mRNA molecule. In higher eukaryotes, an mRNA is a more useful predictor of a polypeptide sequence than is a genomic sequence, because the introns have been spliced out. Researchers prefer to use cDNA rather than mRNA itself because RNAs are inherently less stable than DNA and techniques for routinely amplifying and purifying individual RNA molecules do not exist.
The cDNA is made from mRNA with the use of a special enzyme called reverse transcriptase, originally isolated from retroviruses. Using an mRNA molecule as a template, reverse transcriptase synthesizes a single-stranded DNA molecule that can then be used as a template for double-stranded DNA synthesis. cDNA does not need to be cut in order to be cloned.