P53: a protein product made by the p53 tumor-suppressor gene. When detected in large amounts, it is often associated with abnormal function of p53 and loss of cell-cycle control, which can lead to cancer. The odds of having a tumor with high p53 levels were twice as great for African-American women than for white women.

Cyclin E: a protein important for proper control of cell division. High levels of cyclin E can cause unrestrained cell division and are associated with poorer survival. The odds of having a tumor with high levels of cyclin E were four times greater for African-American women than for white women.

Cyclin D: a protein important for proper control of cell division. High levels of cyclin D in many studies are associated with a better chance of survival. The odds of having a tumor with high cyclin D levels were half as great for African-American as for white women.

When researchers reanalyzed these characteristics by comparing tumors from the same stage or from women diagnosed at the same age, many of these aggressive characteristics were still more common among African-American women.

Porter suggested several possible explanations for why tumors from the African-American women exhibit these differences, most of which affect a woman's exposure to the steroid hormones estrogen and progesterone. "One important factor may be differences between the racial groups with respect to reproductive experiences," she said. "The age at which a woman has children and how many children she has could be a factor. Pregnancy results in very high levels of circulating hormones. The question is whether many pregnancies, and therefore extended periods of high hormone levels, at a relatively young age increase the risk for onset of cancer with aggressive characteristics."

Among women aged 45 or younger, African-Americans have a higher risk of developing breast cancer than white women. Yet among older women, whites are more likely than African-Americans to develop breast cancer.

Porter said that one aspect not addressed in this study was whether the aggressive tumor biology they identified in African-American women contributes to poorer survival. Analyses in this group of Atlanta women are under way to determine the relationship of tumor characteristics and survival. Additionally, she and colleagues in the Southwest Oncology Group, a nationwide network of researchers that conducts clinical trials, are beginning a study to address this. Porter will examine many of the cell-cycle-related tumor characteristics analyzed in this study and the survival outcome of black and white breast-cancer patients enrolled in a clinical trial.

"A major benefit of focusing on women in a clinical trial is that we know that all of them received similar treatment," Porter said. "That will enable us to more directly evaluate the correlation between tumor characteristics and survival."

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