An invention is described that permits the regulation of recombination in cells, organisms or appropriate cell-free systems. The invention involves creating fusion proteins between recombinase proteins, or components of recombinase systems, and ligand binding domains derived from nuclear receptors. The fusion proteins show little recombinase activity in the absence of the ligand that binds to the ligand binding domain. Upon binding of the ligand, recombinase activity is induced. The invention provides a practical means to regulate recombination in cells and organisms and, by linking ligand binding to recombination, provides a simple means whereby ligand binding can be measured as recombination achieved.