A landing pad is an engineered chromosomal site into which one or more transgenes can be targeted for integration. It provides a useful research tool for evaluation of new technologies in a highly controlled manner and facilitates the production of high expressing, stable cell lines for industrial scale protein production. Promosome landing pad uses an integrase from the Streptomyces phage phiC31 to facilitate targeted recombination between two different DNA sequences: attB, the bacterial attachment site, and attP, the phage attachment site. This system is attractive as the attB and attP sites are short (< 40-nucleotides each) and the recombination event is extremely efficient. In addition, because the recombination event does not regenerate the attB and attP sites, integration into an attP site is irreversible. Moreover, mammalian cells do not contain an attP site; therefore an attP sequence engineered into a suitable chromosomal location will be used preferentially as a recombination site.

Cell lines containing a landing pad can be used for technical evaluations and may also be suitable for production. The latter depends on how much protein can be expressed compared to cell lines generated by random integration and screening that often contain 5-10 copies of the transgene. If it is necessary to increase protein expression levels from a landing pad, there are several strategies that can be used inclusive pf Promosome's TEEs and the RESCUE technologies to increase the amounts of protein produced from a single copy of the transgene.