A Minnesota scientist said today that he has developed a blood test for chromosome breaks that may identify people with a genetic predisposition to cancer.

Chromosomes are rod-shaped bodies in every human cell. They carry elements called genes, which govern the cell's activities and convey hereditary characteristics. Certain normal genes somehow change into so-called "oncogenes," which can trigger cancer. There is evidence that chromosome breaks may be involved in causing cancer if they occur at or near sites occupied by oncogenes.

The new test shows whether a person's chromosomes have weak spots that are unusually vulnerable to breakage. Many of the breaks detected by the test are at the sites of these potentially cancer-causing genes.

Dr. Jorge J. Yunis, University of Minnesota cancer researcher, said the test has already revealed that chromosome breaks are less likely to occur in people who take in adequate amounts of folic acid, a B-complex vitamin.

Yunis stressed that there is no evidence yet that folic acid prevents cancer. It is clear only that it reduces the likelihood of breaks detected by the blood test.

The announcement at a news conference -- a somewhat unusual step for research at such a preliminary stage -- coincided with the publication of Yunis' findings in the Dec. 7 issue of the journal Science, which appears today.

On hearing of the report, other cancer researchers reacted cautiously, stressing that much more research is needed before scientists can reach any broad conclusions about the role of chromosome breaks in cancer or the value of folic acid in preventing them.

"I never get excited about these things," said Michael Wigler, a pioneer in cancer gene research at the Cold Spring Harbor Laboratory on Long Island.

"It's always possible to have a genuine breakthrough in this field," Wigler said, "but it's a little like the Messiah. You hear a lot of people making claims but, at best, there's only one."

Yunis said he called the news conference in the hope that publicity would stimulate more research in this area.

Yunis said his test revealed two levels of susceptibility to chromosome breakage.

"All of us," he said, "have 51 weak points in our chromosomes that tend to break when cells are low in folic acid."

In addition, he said, some people have chromosomes that are much more sensitive and break much more readily. This greater sensitivity showed up in people from families with a hereditary tendency to cancer, he said.

Yunis said that although his test would not become generally available for some years, it could eventually be used to identify people at high risk. These people could then try to minimize their exposure to smoke and other cancer-causing substances.

"If our studies are confirmed," Yunis said, "it may be possible to prevent or decrease cancer with a simple vitamin."

Yunis suggested that people found to be at high risk could take folic acid pills, which are available only with a prescription.

People with normal levels of susceptibility, he said, might do well to eat foods naturally high in folic acid. These include leafy vegetables, asparagus, whole grain breads and cereals, fruits, and liver.

Yunis is well known among cancer researchers for his work on chromosome breakage and for identifying specific "fragile sites" where the breaks are likely to occur.

Breakage at fragile sites occurs when cells divide. The pieces of chromosome in the new cells may then rejoin in the wrong order.

One theory as to how this causes cancer is that the rejoining may place a regulator gene next to another gene that is supposed to be dormant or to operate at a low level. The combination may spur the normal gene into abnormal rates of activity.

There is evidence that when this happens to certain genes, the cell suddenly acquires the ability to multiply, proliferating endlessly and growing into a cancer. Molecular biologists now know of about 20 normal human genes that can turn cancerous, becoming oncogenes -- named from the Greek onc for tumor.

Last year Yunis published evidence that nearly all cells from certain kinds of human tumors had sustained chromosomal breaks and rearrangements of the pieces.

He found that tumors of the same type but from different patients often had breaks at the same points in the chromosomes, the fragile sites that Yunis' test identifies.