General Cre-LoxP deletion system. (A) Cre and LoxP system; 34-bp LoxP sequence consisting of two 13-bp inverted and palindromic repeats and 8 bp of core sequences; Cre recombinase is a 38-kDa DNA recombinase. (B) General mechanism of Cre-LoxP system; Cre recombinase recognizes the specific DNA fragment sequences between the two repeated LoxP sites and mediates site-specific deletion of DNA to create two pieces of DNA. (C) Traditional Cre-LoxP mouse generation;two strains of mutant mice are bred to generate Cre-LoxP mice: a Cre-driver mouse strain which has Cre recombinase with a promoter, and a LoxP floxed mouse strain that has LoxP floxed critical exons of the target gene; breeding these two strains together generates heterozygous F1 founder mice; F1 mice then breed with LoxP mice again for the F2 homozygotes.