Estrogen is a hormone that is dominant in the female reproductive system. In women, most estrogen is produced by the ovaries. Men produce estrogen by converting testosterone into estrogen. Because this hormone also has many beneficial effects on brain cells, it currently is being studied as a treatment for Alzheimer's disease. The enzyme that forms the neurotransmitter acetylcholine is promoted in the presence of estrogen. Several very small clinical studies have demonstrated improvement in cognitive function and mood measures in women with Alzheimer's disease who take estrogen.