Bacterial artificial chromosomes (BACs) are DNA molecules assembled in vitro from defined constituents and are stably maintained as one large DNA fragment in Escherichia coli. Artificial chromosomes are useful for genome sequencing programs, for transduction of DNA segments into eukaryotic cells, and for functional characterization of genomic regions and entire viral genomes such as cytomegalovirus (CMV) genomes. CMV genomes in BACs are ready for the advanced tools of E. coli genetics. Homologous and site-specific recombination, or transposon-based approaches allow for the engineering of virtually any kind of genetic change.