Bacterial artificial chromosomes (BACs) represent a very useful cloning system for large DNA fragments and utilize the Escherichia coli F factor as their backbone. E. coli F factor allows strict copy number control of the clones so that they are stably maintained at one to two copies per cell (1 ). The stability of the cloned DNA during propagation in E. coli host is substantially higher in lower copy number vectors than in multi-copy counterparts.