The fundamental problem was that in the presence of the RDF, the recombinase action becomes bidirectional, since either the recombinase on its own or the recombinase-excisionase complex is able to flip DNA, each in a different direction

Although there aredozens of serine integrases that have been described in the literature, there are onlyseven published RDFs for serine integrases (C31 gp3 [22],BT1 gp3 [25], Bxb1 gp47[23], TP901 ORF7/Xis [24],Anabaena/NostocXisI[28], SPBc SprB [27], andRv1 Rv1584c/Xis [26])

for specific stimulation of attL × attR recombination; often called Xis (‘excisionase’) for λ integrase and its tyrosine integrase relatives, or RDF (Recombination Directionality Factor) for serine integrases