To Create a Knockout Mouse, After Introducing to ES Cells

To create a knockout mouse, after introducing to ES cells a DNA construct in which a specific gene is mutagenized by the insertion of a drug resistance marker, the cells

A) that have incorporated the transgene by homologous recombination grow in the presence of the drug and these are injected into a host blastocyst.
B) that lack the transgene grow in the presence of the drug and these are injected into a host blastocyst.
C) that have deleted the transgene grow in the presence of the drug and the nuclei are injected into a host blastocyst.
D) that lack the transgene grow in the presence of the drug and the nuclei are used for nuclear transfer.