The present invention relates to recombinatorial substrates which include a promoter, a terminator, a gene positioned 3' to the terminator and whose expression is to be controlled, and recombination sites on each side of the terminator such that when the substrate is treated with a specific recombinase the gene will be expressed. Recombinatorial substrates which have a promoter, a gene to be controlled, and recombination sites on each side of the gene which when treated with recombinase delete the gene are also provided. Also enclosed are methods of creating transgenic mammals carrying the recombinatorial substrate and methods for activating the recombinatorial substrate.