Human hepatocyte generation in the fetal sheep model is not caused by cell fusion. In situ hybridization of liver sections of (A) a control sheep hybridized with human pericentromeric repeat (red) and sheep pericentromeric repeat (green) probes, and (B) control human liver also hybridized with both probes. As can be seen, the probes were completely species specific. Control sheep displayed only the green signal from the sheep probe, whereas the control human liver exhibited only red fluorescence from the human probe. (C) In situ hybridization of a representative liver section of a chimeric sheep. Human hepatocytes within the sections exhibited pure red fluorescence, demonstrating that they contain only human DNA and not sheep DNA, whereas the sheep hepatocytes exhibit only green fluorescence.