A conventional homologous recombination approach in mouse embryonic stem (ES) cells was used for the targeted insertion of the tetracycline-controlled transactivator (tTA) into intron 1 of the endogenous eukaryotic translation elongation factor 1 alpha 1 (Eef1a1) locus. The tTA is preceded by a splice acceptor site and a floxed transcriptional Stop sequence (i.e., promoterless neo with three polyA signals). (J:292860)