A cDNA library is a collection of expressed DNA genes that are seen as a useful reference tool in gene identification and cloning processes. cDNA libraries are constructed from mRNA using RNA-dependent DNA polymerase reverse transcriptase (RT), which transcribes an mRNA template into DNA. Therefore, a cDNA library can only contain  inserts that are meant to be transcribed into mRNA. This process relies on the principle of DNA/RNA complementarity. The end product of the libraries  is double stranded DNA, which may be inserted into plasmids.  Hence, cDNA libraries are a powerful tool in modern research.