Basic science breakthroughs that had no practical applications in the 1980s are becoming essential to expanded cancer prevention in the 1990s.

That became clear this week as more than four dozen top cancer researchers disclosed their newest work at a science writers' seminar here sponsored by the American Cancer Society.There were no quick, practical applications when laboratory scientists uncovered the oncogene in the mid-1980s, discovering that virtually all cancers are caused by rogue genes that spur abnormal growths.

Now it's clear that different defective genes create the various types of cancers.

The question today: Will unlocking the genetic secrets of this dreaded disease save lives? In some types of cancer, the clear answer is yes - and soon.

One example: Ovarian cancer, which will afflict an estimated 20,300 American women this year, more than 60 percent of whom will die.

"We have found a definite linkage between hereditary breast and ovarian cancer syndrome and a gene on the long arm of chromosome 17," says Dr. Henry Lynch, chairman of the preventive medicine department at Omaha's Creighton University. The chromosomes present in the nucleus of all cells determine physical traits of every living thing.

"This means that for the deadliest of the gynecologic cancers and one that usually defies effective surveillance, in a certain set of families who are prone to getting it, we can now tell through the genetic linkage who will actually get it," Lynch says.

Soon, women in families with a history of breast and ovarian cancer will need only a blood test to see whether they carry the deadly gene.

If so, Lynch says, they can choose to undergo prophylactic operations to remove their ovaries after they've had all the children they plan to bear.

Lynch says the blood test examining chromosome 17 turned up the deadly gene in 60 percent of women tested who were already afflicted with the cancer.

"This means we still have work to do," he said. "It means there may be other genes yet to be found that also cause ovarian cancer."

Gene tests now can also determine which women who experience cancer in one breast are most likely to suffer a repeat on the other side. Women whose tests show they're likely to be two-time victims can choose to have one or two breasts removed to take themselves out of future danger.

A combination of careful tracking of family histories and gene testing also promises to give early warning to parents of children likely to suffer some childhood cancers.