A genomic library is a collection of DNA obtained from the entire genome of a single organism. This DNA is stored in a population of identical vectors, each of which contains a different DNA insert. To build a genomic library, DNA is extracted from cells and then digested with restriction enzymes to cut DNA into fragments of a certain size. Fragments are then inserted into the vector using DNA ligase. Further, the DNA vector can be integrated into the host organism, usually into a population of Escherichia coli or yeast, where each cell contains copies of the vector with one, unique insert. If necessary, individual bacterial or yeast clones containing DNA fragments with the genes of interest or other elements of the genome are isolated and cloned (multiplied). Cloned parts of the genome are isolated from cells and used to solve various theoretical and practical problems of genetics, medicine (including the diagnosis of hereditary diseases) and biotechnology, as well as for genome mapping.