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Scientists have reported discovery of a protein that can block breast-cancer growth in the test tube, a finding that may open new ways of treating and preventing the cancer that strikes one in 10 American women.

University of Michigan researchers said all women appear to have the natural protein, called mammastatin, in their blood, but individual levels vary widely and may possibly be too low in breast-cancer patients.Dr. Max Wicha, director of the university's Cancer Center, said his research team found that mammastatin "is a potent inhibitor of breast-cancer cells growing in the laboratory."

Mammastatin had no effect on growth of 11 other types of cancer cells - a finding that sets it apart from previously discovered cell growth inhibitors and underscores its probable significance in breast cancer, researchers said.

Further tests found mammastatin was present in more than 90 percent of normal human breast cells, but signs of the protein were detected in only 10 to 15 percent of laboratory-grown cancerous breast cells.

The Michigan scientists emphasized their findings, published in the journal Science, do not conclusively link mammastatin levels and breast-cancer development in women. Researchers said they plan to analyze blood samples of many women with and without breast cancer over the next few years to see if such a link exists.

"We speculate that women with low levels of mammastatin may be more prone to develop breast cancer. If this is the case, a scientist could potentially detect and correct a woman's predisposition to breast cancer before the disease occurs," Wicha said.

Cancer is characterized by the abnormal, uncontrolled growth of cells, which invade and destroy healthy tissue.

About one in 10 American women will develop breast cancer during her life. More than 40,000 women are expected to die of breast cancer in 1989 - making it the second leading cause of cancer death among females after lung cancer, the American Cancer Society said.

Dr. John Stevens, the cancer society's vice president for research, said mammastatin "could be of great importance as one piece in the puzzle (of breast cancer), but given the complexity of that puzzle it is unlikely it will be the total answer."

The Michigan researchers hypothesize that most mammastatin is produced at the end of pregnancy, when it acts to shut off the breast cell growth that takes place before a baby is born. It is already known that women who have never given birth are at higher risk of developing breast cancer than those who have.

"If the protective effects of pregnancy are due to mammastatin, then one could potentially administer this agent to lower the risk of developing breast cancer in women who have not been pregnant," Wicha said.

The discovery of mammastatin may also lead to better treatments for women who already have breast cancer.