Bonnet, J., Yin, P., Ortiz, M. E., Subsoontorn, P. & Endy, D. Amplifying genetic logic gates. Science 340, 599–603 (2013).

In this publication, Bonnet and colleagues describe the use of two recombinases to genetate logic gate behaviour.These proteins can flip or excise DNA sequences flanked by their attachment sites (attP and attB). These two are widely used recombinases that complement another collection featured in this database.
Although the authors showed the proteins do work, they did not develop a simple metrology to describe their efficiency. For this reason and to maintain consistency along the database, no quantitative values are attached to these parts.