**1.1 A brief history of BACs**

The BAC vector that replicates in *Escherichia coli* K-12 was first introduced in 1992 by Shizuya et al. More detailed information on current BAC applications may be referred to other chapters of this book. Because BACs can accommodate much longer DNA stretches than the plasmids for *E. coli* available at that time, as large as >100 kbp, they were suitable for the preparation of DNA libraries covering entire cellular genomes regardless of their original size (Fig. 1a). The initial goal for BACs was to provide materials for whole genome-sequencing projects and

Fig. 1a. BAC library for the contiguous genomic DNA. The ability to carry large DNA (>100kb) is useful for not only the direct analysis of genomic regions in reverse genetics, but also for the *de novo* synthesis of genomic DNA in the field of synthetic biology.

their contribution to long-range sequence determinations has been demonstrated (Frengen et al., 1999; Osoegawa et al., 2001). Cutting-edge technologies that facilitate direct genome sequencing have dramatically reduced the need for BAC libraries as a sequencing resource.
