Gene Disruption - Homologous recombination has been exploited to produce 'knockout' mice in which specific gene functions have been knocked out by disrupting or removing the concerned genes present in their genomes. The gene, say, gene A, to be disrupted is isolated, cloned and a selectable marker gene, e.g., the bacterial gene neo, is placed within this gene.

The neo gene disrupts the cloned gene A, as well as makes the cells in which it is integrated resistant to the antibiotic G418. Another gene like HSV-tK (Herpus simplex virus thymidine kinase) is linked to this gene construct. HSV-tK makes the mice cells containing it sensitive to the nucleoside analogue gancyclovir; thus it permits a negative selection for such cells.

Such ES cells will lose gene A function, will become resistant to G418 and will be resistant to gancyclovir since they will not acquire the gene HSV-tK.

But in cases of random integration, the entire gene construct will be integrated in the mouse chromosomes; such cells will be G418 resistant (due to neo gene) and sensitive to gancyclovir due to the HSV-tK gene; they will also have their gene A intact.

Therefore, when transfected ES cells are grown on a medium containing G418 and gancyclovir, only those cells in which homologous recombination has taken place will survive; in all such cells gene A will be disrupted by the gene neo. The selected ES cells are introduced into young mice embryos to produce chimaeric mice in which gene A function has been knocked out in the ES cell derived tissues.