Figure 1.

The Cre–LoxP system for controlled gene knockout. (A) The gene of interest is flanked by LoxP sites (“floxed”) in non-coding regions of the genome through gene targeting. The floxed gene should ideally be normally expressed and functional, so that the gene-targeted mice are phenotypically wild-type. Expression of Cre recombinase induces recombination between the LoxP sites, thus knocking out the floxed gene. (B) Schematic drawing of a part of the somatosensory mouse barrel cortex. The barrels in layer 4 are outlined in cyan, defining the lateral boundaries of a cortical column. Pyramidal neurons are indicated by triangular cell bodies with ascending apical dendrites. Interneurons are depicted as diamonds with local dendritic arbors. In this schematic drawing, pyramidal neurons of layer 2/3 of the central barrel column express Cre (green in this depiction) and the floxed gene would be eliminated in these cells. Neighboring non-pyramidal neurons are wild-type. Neurons in other cortical columns and in other layers are also wild-type. Therefore, a highly controlled knockout of a floxed gene will be induced if Cre recombinase is expressed in a specific cell type of a single layer of one cortical column.