In human studies, Ragonese et al. [143] found that PD was associated with factors that reduced estrogen stimulation during life (e.g. PD was significantly associated with a fertile life length shorter than 36 years and a cumulative length of pregnancies longer than 30 months). Currie et al. [144] also reported that women who had taken postmenopausal estrogen replacement therapy were less likely to develop PD than those who had not. Furthermore, a single nucleotide polymorphism allele of the estrogen receptor-_ gene was recently shown to be more frequent in patients that have an early age of onset of PD [145]. Of significant interest and paralleling results in animal studies, a pilot study inwomen revealed that short term estrogen treatment in postmenopausal women increased dopamine transporter availability in the caudate putamen [146].