Fig. 2.

Conditional gene knockout. Gene knockout models can be generated using the FLP-Frt or Cre-LoxP systems. This requires the generation of two constructs: 1) a construct containing Frt or LoxP recognition sites inserted into the intron sequences flanking the genomic region to be knocked out; and 2) a construct containing a FLP or Cre recombinase under the control of a tissue-specific promoter. These constructs are introduced into two different mouse strains using knockin/transgenic over-expression methods, to generate one mouse expressing the Frt/LoxP flanked genomic sequence in all tissues, and one mouse expressing FLP/Cre recombinase in a specific organ e.g. pituitary. These two mouse lines are then bred to generate a mouse containing both the Frt/LoxP flanked genomic sequence and the tissue-specific recombinase. In FLP/Cre-expressing tissues e.g. pituitary, the FLP/Cre binds to its target Frt/LoxP sites and catalyses recombination of the DNA, leading to excision of the genetic material contained between the target sites.