cDNA Library From mRNAs - Complementary DNA (cDNA) libraries can also be prepared by isolating mRNAs from tissues which are actively synthesizing proteins, like roots and leaves in plants, ovaries or reticulocytes in mammals, etc. The mRNA is used for copying it into cDNA through the use of reverse transcriptase.

A cDNA molecule can be made double stranded, and cloned and discussed later. However cDNA clones will differ from genomic clones in lacking the introns present in split genes, but may have the advantage of being capable to be expressed in bacteria, which do not have the machinery to process the HnRNA (obtained from split genes) into mRNA.