The present invention relates to a metazoan organism, with the exception of humans, and in particular a mouse, characterized in that at least one cell of this organism comprises at least one fusion protein between a recombinase Cre and a modified ligand binding domain of the nuclear estrogen receptor alpha, allowing the inactive fused recombinase to be induced by synthetic antiestrogens, but not by natural estrogens, and one or more DNA sequences of interest belonging to the genome of said organism into which one or more sites of recognition of said recombinase protein are inserted. The invention also covers the methods using said organism for the screening of medicaments, the mutagenesis and the analysis of the biological function of the DNA sequences) of interest, in particular of gene(s) of interest, such as RXR.sub..alpha..