To isolate a specific gene, it is often necessary to produce a genomic library, since the number of genes present in a genome can be very large.  Generally, a genomic library is composed of many self-replicating plasmid vectors, each containing a genetic insert from the genome of interest.

The four most common cloning vectors used to represent the haploid human genome are listed below:

Vector Average Insert (bp) Clones to Represent Genome
Plasmid 4000 750,000
Lambda (l) 20,000 150,000
Cosmid 40,000 75,000
Yeast Artificial Chromosome (YAC) 400,000 7,500
Modified forms of bacteriophage l are generally used for library construction, since they can accomodate inserts averaging 20,000 base pairs (1/5 as many copies needed as compared to plasmid vectors), and efficiency of recombination of E. coli is much greater via transfection (phage infection) than by artificial transformation.

Cosmid (combination l/plasmid) vectors and YAC vectors are also being used more commonly, since each can accomodate even larger inserts and require fewer copies to represent the entire genome.