Practice question

Positive-negative selection strategy identifies homologous recombination. Neomycin resistance inside homology arms provides positive selection; only vector integrated cells survive G418. HSV-tk outside arms provides negative selection; random integrants retain tk and convert ganciclovir to toxic triphosphate. True homologous recombinants lose tk through double crossover. Ideal phenotype is G418 resistance due to neo inclusion and inability to grow under ganciclovir pressure indicating tk presence elimination, often reported as G418 resistant and ganciclovir sensitive in screening nomenclature, allowing enrichment of targeted embryonic stem cells.