Estrogen mimics are a group of different molecules that do not have any obvious structural similarities. What most of these molecules do is attach themselves to estrogen receptors in cells and mimic the action of the body's natural estrogen. Or, after attaching to estrogen receptors, they may block the action of natural estrogen and are thus called estrogen antagonists. Estrogen mimics may also interfere with the normal metabolism of estrogen in the body. The amount of natural estrogen in a woman's body is normally much greater than the amount of estrogen mimics. However, most of the natural estrogen is bound by sex-hormone-binding protein in the blood, which is incapable of binding estrogen mimics. Thus, estrogen mimics are free to bind to receptors, so that, in effect, their concentration in the body is higher than their measured concentrations would suggest. It is hypothesized that the estrogen mimicing properties of DDT and its metabolites may be responsible for a decrease in men's sperm counts since the 1940's, as well as for the increase in the number of cases of endometriosis (the growth outside the uterus of cells that normally line the uterus; it often leads to infertility in women).