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Cre and the LoxP sites
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Cre Recombinase and the LoxP Sites
The sites that the Cre recombinase protein recognizes are called loxP sites. LoxP stands for locus of crossover (x). The loxP sites are 34 base pair sequences. They have an 8 base pair directional spacer sandwiched between two 13 base pair palindromic flanking sequences.
The variables that determine how Cre recombinase recombines the DNA are the location and orientation of the loxP sites. The sites can either be cis, meaning on the same peice of DNA, or trans, meaning on different peices of DNA. Then the direction of the spacers can be the same or opposite. For cis recombination the if the spacers are in the same orientation the excision of DNA between the sights occurs and if the spacers are in opposite orientation inversion occurs. If the loxP sites are trans and in the same orientation the insertion of one gene into the other occurs. If in the opposite orientation translocation occurs [4].