text: A cDNA library represents a sample of the mRNA purified from a particular source (either a collection of cells, a particular tissue, or an entire organism), which has been converted back to a DNA template by the use of the enzyme reverse transcriptase. It thus represents the genes that were being actively transcribed in that particular source under the physiological, developmental, or environmental conditions that existed when the mRNA was purified. cDNA libraries can be generated using techniques that promote "full-length" clones or under conditions that generate shorter fragments used for the identification of "expressed sequence tags".
  cDNA libraries are useful in reverse genetics, but they only represent a very small (less than 1%) portion of the overall genome in a given organism.
  Applications of cDNA libraries include:
  * Discovery of novel genes
  * Cloning of full-length cDNA molecules for in vitro study of gene function
  * Study of the repertoire of mRNAs expressed in different cells or tissues
  * Study of alternative splicing in different cells or tissues