Figure 1

Representation of genetic mouse models. 1) Knockout out of specific genes. A neomycin cassette is inserted by homologous recombination either into an exon (1A) of a gene of interest or replacing the whole gene of interest (1B). 2) Methods to control the cell specificity and timing of gene deletion. Homologous recombination is used to insert lox-p sites surrounding an exon of the gene of interest. To conditionally delete the gene in a specific cell type, the Cre recombinase is expressed by a tissue specific promoter and deletes the loxP flanked region (2A). To regulate the timing, one can use a transgene encoding a Cre recombinase fused with the modified estrogen receptor (Cre-ERT) that will move into the nucleus upon injection of tamoxifen (2B). 3) Ectopic expression of a transgene. A common method is to utilize a lox-stop-lox cassette which can be removed by Cre recombinase. A transgene is generated with a stop codon that is flanked with two loxP sites upstream of the transgene of interest. The transgene can be introduced to the genome at a specific locus by homologous recombination or randomly inserted in the genome. When the Cre recombinase deletes the stop codon, the transgene can be transcribed (3A). This technique is available with the Cre-ERT system (3B). 4) To reversibly express a transgene. A common method is the use of the TRE/tTA or TRE/rtTA systems. A transgene is generated with the gene of interest downstream of the tetracycline responsive element (TRE). A second transgene is generated with a tissue specific promoter controlling the expression of tTA (Tet-Off, 4A) or rtTA (Tet-ON, 4B). For Tet-OFF, the tTA activates transcription of the transgene downstream of the TRE promoter, only in the absence of doxycycline. For Tet-ON, the rtTA activates transcription of the transgene downstream of the TRE promoter only in the presence of doxycycline.