Thus, both animal and human studies suggest a beneficial regulatory effect of estrogen on the nigrostriatal dopaminergic system, which may underlie putative protective effects of estrogen in PD. A corollary and exciting finding was the recent discovery that estrogen treatment induces differentiation of human neural stem cells (NSCs), giving rise to tyrosine hydroxylase (dopaminergic) neurons, an effect blocked by use of an estrogen receptor antagonist [147]. Estrogen also increased the number of dopaminergic neurons derived from human NSCs in vivo when the cells were grafted into mouse brains. This finding reveals another layer to the beneficial regulation by estrogen of the dopaminergic neuronal system, and raises the possibility of a potential role for estrogen in transplantation of NSCs for PD. Finally, oxidative stress is suggested to play an important role in neuronal degeneration in PD [148]. It is thus intriguing that estrogen has been shown to suppress free radical production and protect striatal neurons against oxidative stress [111–113], potentially providing an additional mechanism for estrogen neuroprotection in PD.