Human artificial chromosomes (HACs): Human artificial chromosomes (HACs) or mammalian artificial chromosomes (MACs) are still under development. HACs are microchromosomes that can act as a new chromosome in a population of human cells. HACs range in size from 6 to 10 Mb that carry new genes introduced by human researchers. HACs can be used as vectors in transfer of new genes, studying their expression and mammalian chromosomal function can also be elucidated using these microchromosomes in mammalian system. Different types of vectors are summarized in Table 1.

Table 1: Different types of vector with their properties.

Vector	Basis	Size limit of insert	Major application
Plasmid	Naturally occurring multi copy plasmids	≤ 10 kb	Subcloning and gene manipulation, cDNA cloning and expression studies.
Phage	Bacteriophage λ	10-20kb	Genomic DNA cloning, cDNA and expression libraries.
Cosmid	Plasmid containing a bacteriophage λ cos site	35-45 kb	Genomic library construction.
BACs	Escherichia coli F factor plasmid	75-300 kb	Analysis of large genomes.
YACs	Saccharomyces cervisiae centromere, telomere, and autonomously replicating sequence	100- 3000 kb	Analysis of large genomes, YAC transgenic mice.
MACs	Mammalian centromere, telomere, and origin of replication	4> 10 mb	Still in budding stage for use in animal biotechnology and human gene therapy.