A major new cancer finding is making the rounds in the scientific world, and its conclusion is downright groundbreaking: a huge number of cancers may be caused by simple bad luck, rather than because of adverse actions like smoking or bad genes. The new study, which was published on Thursday, states that most mutations that lead to cancer occur naturally and can’t be prevented.

That doesn’t mean things like smoking or genetics don’t contribute to cancer, but many of them will happen whether or not you have great genes and live a healthy life. While scientists had long suspected that luck played a role, this study shows just how big that role is, and that many cases of cancer are totally unavoidable.

Normal cells divide all the time, and each time they do they make a few mistakes in the DNA copies. Usually, these happens to bits of DNA that are unimportant, but very rarely, they happen to a gene that is considered a cancer driver.

“Most textbooks attribute cancer-causing mutations to two major sources: inherited and environmental factors,” the paper’s abstract states. “A recent study highlighted the prominent role in cancer of replicative (R) mutations that arise from a third source: unavoidable errors associated with DNA replication. Tomasetti et al. developed a method for determining the proportions of cancer-causing mutations that result from inherited, environmental, and replicative factors (see the Perspective by Nowak and Waclaw). They found that a substantial fraction of cancer driver gene mutations are indeed due to replicative factors. The results are consistent with epidemiological estimates of the fraction of preventable cancers.

“Cancers are caused by mutations that may be inherited, induced by environmental factors, or result from DNA replication errors (R). We studied the relationship between the number of normal stem cell divisions and the risk of 17 cancer types in 69 countries throughout the world. The data revealed a strong correlation (median = 0.80) between cancer incidence and normal stem cell divisions in all countries, regardless of their environment. The major role of R mutations in cancer etiology was supported by an independent approach, based solely on cancer genome sequencing and epidemiological data, which suggested that R mutations are responsible for two-thirds of the mutations in human cancers. All of these results are consistent with epidemiological estimates of the fraction of cancers that can be prevented by changes in the environment. Moreover, they accentuate the importance of early detection and intervention to reduce deaths from the many cancers arising from unavoidable R mutations.”