A genomic DNA library is made from the genomic DNA of an organism. For instance, a mouse genomic library could be made by digesting mouse nuclear DNA with a restriction nuclease to make a huge number of several DNA fragments but all with identical cohesive ends.

           Figure:  A simple method of DNA cloning using a plasmid vector

The DNA fragments would then be ligated into linearized plasmid vector molecules or into an appropriate virus vector.  This library would hold all of the nuclear DNA sequence of the mouse and could be searched for any ceratin mouse gene of interest.  Each clone in the library is called as a genomic DNA clone. Not each genomic DNA clone would hold a complete gene since in various cases the restriction enzyme will have cut at least once within the gene. Thus some clones will have only a category of a gene.