These cells are resistant to neomycin but killed by gancyclovir. Step 2 Positive selection of transformed cells by culturing all cells in neomycin. Step 4 Inject these into the inner cell mass of mouse blastocysts. Many genes turn out not to be indispensable. The mouse genome appears to have sufficient redundancy to compensate for a single missing pair of alleles.
Most genes are pleiotropic .
They are expressed in different tissues in different ways and at different times in development. Until recently, the transgenes introduced into sheep inserted randomly in the genome and often worked poorly. However, in July 2000, success at inserting a transgene into a specific gene locus was reported. The gene was the human gene for alpha1-antitrypsin and two of the animals expressed large quantities of the human protein in their milk. This is how it was done.