Endometrial cancer remains one of the significant problems associated with TAM. A TAM metabolite binds to DNA and forms DNA adducts which damage cells. It is reported that TAM has an expanded ability to form DNA adducts compared with TOR in vitro. A recent study compared endometrial cells collected from patients in which TAM or TOR had been administered. The k-ras gene mutation was investigated in these cases, and it showed that TAM held a higher frequency of gene mutation. Although we still need to discuss whether or not k-ras mutation is directly related to the development of endometrial cancer, TAM seems to have a higher risk of inducing cancer compared with TOR.