Estrogens are hormones, which means that they function as signaling molecules. A signaling molecule exerts its effects by traveling through the bloodstream and interacting with cells in a variety of target tissues.
The breast and the uterus, which play central roles in sexual reproduction, are two of the main targets of estrogen. In addition, estrogen molecules act on the brain, bone, liver, and heart.
![Estrogen Target Tissues](https://webarchive.library.unt.edu/eot2008/20090117100808im_/http://www.cancer.gov/images/Documents/687af7a8-0649-40dc-b571-c4b112b81448/cancer2.jpg)
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