sentences:  Tyrosine recombinases, such as Cre or FLP, cleave one DNA strand at a time at points that are staggered by 6–8bp, linking the 3’ end of the strand to the hydroxyl group of the tyrosine nucleophile (Fig.1).Strand exchange then proceeds via a crossed strand intermediate analogous to the Holliday junction in which only one pair of strands has been exchanged.