Why don't you use mouse ES cells?

Mouse ES cells have similar properties as human ES cells and can give rise to all cell types of the mouse. However, for toxicological assays, physiological differences between mouse and human cell types can mean that a substance might be tested negative in the mouse system whereas it has significant toxicity in humans. To avoid such potentially dangerous situations and exclude interspecies differences, the ESNATS project will use human embryonic stem cells for its studies.