Looking for genes that could boost a person's general risk for cancer is not likely to reap great rewards, new research concludes.

While vast sums of money and resources are being invested in the search for common, inherited genetic variants that increase susceptibility to cancer, that search faces big roadblocks.

The biggest one: Such genes probably don't exist, scientists now say. And if they do, they probably won't have much effect on the incidence of cancer.

"Enthusiasm for this new field of research should not precipitate unwarranted expectations," wrote experts Dr. Stuart Baker, from the U.S. National Cancer Institute, and Dr. Jaakko Kaprio, from the University of Helsinki, Finland.

Their report appears in the May 13 issue of the British Medical Journal.

They pointed to studies that suggest that environmental, dietary or lifestyle changes have a much larger effect on the incidence of cancer than genetics do. Those studies find changes in the incidence of cancer within one generation or two generations, which is just too quick to be related to the introduction of new genes, Baker and Kaprio noted.

A pivotal study of cancer in twins also suggests genes aren't the key culprits in cancer. Looking at that data from identical and non-identical twins, that study found that genetic susceptibility had only a small-to-moderate effect on the incidence of cancer.

Baker and Kaprio believe that studies that have shown a strong association between specific genes and a higher risk for cancers may be biased.

"The search for common cancer susceptibility genes faces important methodological and practical challenges for cancer prevention, given the small chance that such genetic variants exist and the difficulty and expense of proving substantial clinical benefit if they do exist," Baker and Kaprio wrote.

The researchers noted that certain genes may enhance risks for very specific types of cancer -- for example, the BRCA1 and BRCA2 genes that doctors know are strongly linked to breast cancer. But genes that encourage cancer generally are less likely, they said.

One expert agreed that genetics can only go so far in determining anyone's risk for cancer.

"The age of the human genome is here," said Dr. Michael Thun, vice president of epidemiology and surveillance research at the American Cancer Society. "The understanding of genes will transform the way we think about many diseases and is already transforming the way we understand cancers."

But the idea that that your genetic makeup is going to be the main factor that determines your susceptibility to cancer has been oversold, Thun added. "The things we know that are bad for you are bad for you in so many different ways that they won't become OK just because you're less susceptible to one or another disease," he said.

Genetics isn't going to transform efforts to prevent cancer, Thun added. "The most successful efforts to prevent cancer are going to come from public policies that make it easier to maintain a healthy body weight and make it easier to get kids not to smoke," he said.

Most of the genetic changes that cause cancer happen during your lifetime, not at birth, Thun said. Still, "research on what the genetic changes are that give rise to a cancer is already profoundly important," he said.