4-Hydroxyestrone (4-OHE1) is a carcinogenic metabolite originating from 17beta-estradiol (17β-E2) hormone. 4-OHE1 was a partial estrogen agonist on cancellous bone turnover, similar to 17β-E2, is able to emit electrons from its excited singlet state. The quantum yield is nearly the same like this of 17β-E2 and decreases with increasing substrate concentration, but enhances with temperature. The 4-OHE1 transient derived from the electron ejection process is leading to the formation of a secondary metabolite by reaction with the present C2H4OH radical. This fact indicates, that depending on the life habits of a person, a variety of metabolites can be formed, also such initiating cancer.