A new study in Cancer Research from Loyola University Chicago Stritch School of Medicine researchers is providing new clues as to how estrogen fuels many breast cancers. The researchers have found that the hormone inhibits the MLK3 protein that causes normal cell death. When the protein is blocked, cancer cells can grow uncontrolled, and can be resistant to chemotherapy. The researchers found that in estrogen-positive or progesterone-positive cancer cells, MLK3 activity was reduced. This could give medicine new ways the fight the deadly disease as about 60 percent of all breast cancers are estrogen-positive or progesterone-positive.