A cDNA library is made by using the reverse transcriptase of a retrovirus  to synthesize cDNA (complementary  DNA)  copies  of the total mRNA from a cell (or perhaps a subfraction of it). A single-stranded cDNA is transformed into double-stranded DNA and inserted into the vector.  Every clone in the library is called as a cDNA clone. Unlike a complete genomic library that holds all of the nuclear DNA sequences of an organism a cDNA library have only sequences which are expressed as mRNA. Several tissues of an animal, which express some genes in general but also several different genes, will thus yield different cDNA libraries.