Bacterial artificial chromosome (BAC) is a chromosome-like structure, made by genetic engineering. BAC is a cloning vector able to carrying between 100 and 300 kilo bases of target series. They are propagated as the mini-chromosome in a bacterial host. The size of typical BAC is ideal for use as an intermediate in large-scale genome sequencing projects. The entire genomes can be cloned into BAC libraries, and the entire BAC clones can be shotgun-sequenced fairly rapidly.