Your Student Is Trying to Create a "Gene X" Knockout

Essay

Your student is trying to create a "Gene X" knockout mouse by introducing a recombinant construct into embryonic stem (ES)cells, selecting transformants, and introducing the transformed cells into mouse blastocysts. He designed his construct as follows:
Neo^R----region of homology to Gene X---HSV tk--- region of homology to Gene X
where Neo^R is the neomycin-resistance gene and HSV tk is the Herpes Simplex virus thymidine kinase gene. He transforms the ES cells and exposes them to G418 and ganciclovir. Explain to your student he failed to recover transformed ES cells.