This is based on the integration/excision mechanism of the lambda phage and E. coli bacterium. Integration involves the λ attP site within the bacterial attB site. The bacterial attL and attR sites flank the integrated phage genome, excision reverses the process. Such a procedure can be used for the generation of vectors with DNA-binding and activation domain sites facilitating the study of protein interaction by analogy of the two-hybrid method.  att sites,  two-hybrid method; Muyrers JP et al 2001 Trends Biochem Sci 26[5]:325.