If you smoke and get lung cancer, you can't blame your genes, says a new study by American researchers.

The study of 681 male twins who smoked and died of lung cancer found most of their twin brothers did not, challenging the theory that certain genes make people susceptible to cancer."A smoker might believe he won't get cancer because his parents smoked and didn't get lung cancer. But I believe that's a fatal mistake," said one of the researchers, Dr. M. Miles Braun of the National Cancer Institute in Bethesda, Md.

About 80 percent of smokers do not get lung cancer, though many die of heart disease, diabetes or other smoking-related diseases.

The American Cancer Society predicts lung cancer will kill 153,000 men and women in the United States in 1994. The number of lung cancer deaths is rising all over the world.

The new study surveyed 681 twins born in the United States between 1917 and 1927. All 681 died of smoking-related lung cancer, but in the overwhelming majority of cases, their brothers, who also smoked, did not.

Most of the pairs of twins smoked for about the same number of years and about as many cigarettes a day, Braun said.

Lung cancer killed 282 men with an identical twin, but only 10 pairs. It killed 399 men with a fraternal twin, but only 21 pairs.

"I was somewhat surprised because the hunch was that even for a strong environmental carcinogen such as smoking, there would be some inherited predisposition," Braun said.

The study, done with scientists at the National Academy of Sciences in Washington, D.C., is published in the Aug. 13 issue of The Lancet, a medical journal.