Figure 1. To avoid linking a bacterial promoter to a foreign gene on a plasmid, the promoter and gene are cloned separately, then combined in vitro to form a DNA circle. The circle is integrated into the bacterial chromosome using lambda integrase, and the resulting integrant is plated on tetracycline media to select for tandem duplications of the gene of interest. LB, L-broth agar; CTD, chromosomal transfer DNA. Reproduced from Mascarenhas D (1998) Nature Biotechnology 16: 1371–1372.