**1.2 BACs generation**

BACs can be considered as plasmid expression vectors, amplified in bacteria and composed of a small amount of bacterial DNA derived from the single copy F-plasmid (Shizuya, Birren et al. 1992). F-plasmid has genes required for prokaryotic replication, partition, and selection. Viral BACs are generated by inserting a BAC vector sequence into a viral genome (Warden, Tang et al. 2011). Direct deletion mutants and random transposon mutagenesis of viral BACs are commonly used to determine the function of viral genes (Warden, Tang et al. 2011). Mammalian DNA, human genome or mouse genome (100–350 kb) can also be cloned as BACs vectors. Currently most regions of the human genome and the genome of other species are available as BACs. These vectors are useful tools in human genome-sequencing projects (Lander, Linton et al. 2001; Venter, Adams et al. 2001) and transgenic mice studies. Using endogenous homologous recombination systems in *E. coli* (Copeland, Jenkins et al. 2001), BACs can be modified in many ways by recombination. They can be genetically engineered to express reporter genes, or to put a transgene under a specific promoter which will be more amenable in gene expression in the mammalian cells. BACs can be modified to replace any nucleotide sequence of interest (gene replacement), remove any existing DNA sequence, or introduce new sequences without removing any of the existing sequences (gene removal or insertion). In addition BACs are also used to place nucleotide substitution through selection/counterselection strategy, and to conduct effective gap repair cloning of any target site of interest (Adamson, Jackson et al. 2011).
