A gene library is a collection of different DNA sequences from an organism each of which has been cloned into a vector for ease of purification, storage and analysis. There are essentially two types of gene library that can be made depending on the source of the DNA used. If the DNA is genomic DNA, the library is called a genomic library. If the DNA is a copy of an mRNA population, that is cDNA, then the library is called a cDNA library. When producing a gene library, an important consideration is how well it represents the starting material, that is does it contain all the original sequences (a representative library)? If certain sequences have not been cloned, for example repetitive sequences lacking restriction sites , the library is not representative. Likewise, if the library does not contain a sufficient number of clones, then it is probable that some genes will be missing. cDNA libraries that are enriched for certain sequences will obviously lack others, but if correctly prepared and propagated they can be representative of the enriched mRNA starting material.
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