A new type of gene trap cassette, which can induce conditional mutations, relies on directional site-specific recombination systems, which can repair and re-induce gene trap mutations when activated in succession. After the gene trap cassettes are inserted into the genome of the target organism, mutations can be activated at a particular time and place in somatic cells. The gene trap cassettes also create multipurpose alleles amendable to a wide range of post-insertional modifications. Such gene trap cassettes can be used to mutationally inactivate all cellular genes temporally and/or spatially. Cells which contain the inventive gene trap cassette can be used for identification and/or isolation of genes and for the creation of transgenic organisms to study gene function at various developmental stages, including the adult, as well as for the creation of animal models of human disease useful for in vivo drug target validation.