Bacteriophage: The viruses that infect bacteria are called bacteriophage. These are intracellular obligate parasites that multiply inside bacterial cell by making use of some or all of the host enzymes. Bacteriophages have a very high significant mechanism for delivering its genome into bacterial cell. Hence it can be used as a cloning vector to deliver larger DNA segments. Most of the bacteriophage genome is non-essential and can be replaced with foreign DNA. Using bacteriophage as a vector, a DNA fragment of size up to 20 kb can be transformed.

Bacterial artificial chromosomes (BACs): Bacterial artificial chromosomes (BACs) are simple plasmid which is designed to clone very large DNA fragments ranging in size from 75 to 300 kb. BACs basically have marker like sights such as antibiotic resistance genes and a very stable origin of replication (ori) that promotes the distribution of plasmid after bacterial cell division and maintaining the plasmid copy number to one or two per cell. BACs are basically used in sequencing the genome of organisms in genome projects (example: BACs were used in human genome project). Several hundred thousand base pair DNA fragments can be cloned using BACs.

Yeast artificial chromosomes (YACs): YACs are yeast expression vectors. A very large DNA fragments whose sizes ranging from 100 kb to 3000 kb can be cloned using YACs. Mostly YACs are used for cloning very large DNA fragments and for the physical mapping of complex genomes. YACs have an advantage over BACs in expressing eukaryotic proteins that require post translational modifications. But, YACs are known to produce chimeric effects which make them less stable compared to BACs.