Most mammalian cell types do not undergo homologous recombination (HR), and even mouse ES cells do so at low frequency - about 1/200000 cells. However, if you first introduce a crRNA designed to target a mouse genomic sequence next to a PAM site, plus a source of Cas9 protein and a plasmid with homology arms, most mouse cell types will undergo HR with very high frequency -as high as 1/5 or more of the treated cells. What does this tell you about the mechanism of HR in mammalian cells