The human genome is partially digested with a restriction enzyme like Sau3A in a specific way so that some of the sites are cleaved and others are not. By this way, random cleavage of the sites occurs and a collection of overhanging fragments of length suitable for cloning can be obtained, which are then ligated into bacteriophage lambda “arms” prepared so that the Sau3A ends of human DNA fragments can be ligated into the vector. The recombinant lambda chromosomes are then packaged into the infectious bacteriophage, and then the library, containing 1 million or more fragments of genomic DNA, can be ...