These results could indicate that the reduction in Engrailed expression increases the vulnerability of mesDA neurons to mitochondrial dysfunction in a general manner, since the three most commonly used reagents to model PD – MPP+, 6-Hydroxydopamine (6-OHDA) and rotenone [17,29,49,50] – also cause cell death by induction of this pathway of apoptosis, evident from activation of caspase-9 [51-53] (Figure 7A–C). The gene discussed is CASP9; the disease is Parkinson disease.