10.1 Cre-lox system:

This system allows the genetic manipulation of target cells to control its gene expression, delete specific DNA sequences, or modify the genomic content. The Cre recombinase is a site-specific integrase isolated from bacteriophage P1. It catalyzes the recombination of DNA between specific loxP sites in DNA. Generally, this system is created after generating two strains, one expressing Cre recombinase and the other having loxP site flanked with the gene of interest. Both the strains are crossed in order to allow independent recombination and their outcome is determined by the location and orientation of loxP site containing gene of interest. If the loxP sites are oriented in opposite directions, Cre recombinase mediates the inversion of the gene of interest (Figure 10.1). However, Cre recombinase mediates a translocation event if the loxP sites are located on different chromosomes (Figure 10.2) and deletion event if the loxP sites are oriented in the same direction on a chromosome segment (Figure 10.3).

Some terminology: Pluripotent embryonic stem cells are undifferentiated early embryonic cells derived from the inner cell mass of mouse blastocysts. Nuclear localization sequences (NLS) are important for directing a protein to nucleus. Generally NLS contains PK3RKV amino acid residues in the protein. The proteins are directed to the nucleus with the help of nuclear pore complex and with the combined effect of RAN-GTPase and IMPORTIN molecules.