Estriol is a hormone produced by the placenta that is virtually undetectable until pregnancy, when it progressively increases. It is thought that its role is to suppress a woman's immune system when she is pregnant, so that the fetus will not be seen by the body as a foreign "invader." "The beauty of estriol is that it can be given as a pill, not a shot, and also that it's not a new drug; it has decades of safety behind it," said Voskuhl, who holds the Jack H. Skirball Chair for Multiple Sclerosis in the UCLA Department of Neurology. For years estriol has been in widespread use in Europe and Asia as hormone replacement therapy for women with menopausal symptoms. The fact that the pill already exists, she said, should dramatically reduce the cost of treatment.