ES cells

All ES cells that do not take up the vector
(the majority) are sensitive to neomycin and
will not grow in a medium containing the antibiotic
(positive selection, not shown). Cells that
take up the vector (about 1%) do so either by
nonhomologous insertion at random sites (4) or
by homologous recombination at the target site
(5). These cells can be distinguished when
grown in a selective medium containing gancyclovir,
a nucleotide analogue toxic to cells containing
tk+. Only cells containing the gene-targeted
insertion (through homologous recombination)
will grow because they do not contain
tk+ (negative selection).