Gene knockout by homologous recombination

The vector consists of two homologous regions which have complementary strands of DNA on either side of the targeted gene in the genome. The disrupted gene along with a gene coding for neomycin resistance are included between the homologous regions. A thymidine kinase gene is included outside the homologous region. Once the vector is introduced into the host cell, it can either be incorporated into the genome by homologous recombination (the desired result), undergo complete insertion into the genome at an undesirable point or not be taken up by the cell at all. Neomycin resistance is used to select for cells in which the vector has been taken up and then gancyclovir sensitivity is used to select for cells which have undergone recombination.