Estrogen can be synthesized in various tissues of men and women. Post-menopausal women's circulating estrogen levels are mainly maintained by estrogen synthesized by adipose tissue. However, it has been found that postmenopausal women's mammary gland tissue has estradiol levels 10 times higher than plasma. It has been reported that the activity of aromatase and the mRNA of aromatase are present in both normal breast and breast tumors. Some clinical observations have shown that aromatization in tumors is associated with tumor suppression of estrogen synthesis after treatment with aromatase inhibitors, and local estrogen production may also play an important role in tumor proliferation. However, the aromatase activity measured in human breast cancer tissue homogenates is relatively low and insufficient to catalyze the formation of a sufficient amount of estrogen to activate the estrogen receptor. Other studies have shown that local estrogen concentrations are sufficient to stimulate tumor growth. At the same time, tissue culture found that certain estrogen-stimulated tumors can also accelerate proliferation through testosterone, and this stimulation can be blocked by aromatase inhibitors. It suggests that testosterone is aromatized to produce estrogen. Thus, aromatase in tumors is important for the production of estrogen that stimulates tumor proliferation.