The claim that sparked this controversy? That “bad luck,” more than environmental factors or inherited genes, affects whether someone develops cancer, implying that preventive efforts from smoking cessation to environmental cleanups were largely pointless.

Now the authors of that 2015 paper are back. In a study published on Thursday in Science, they double down on their original finding but also labor mightily to correct widespread misinterpretations of it. This time, using health records from 69 countries, they conclude that 66 percent of cancer-causing genetic mutations arise from the “bad luck” of a healthy, dividing cell making a random mistake when it copies its DNA.

The scientists go to great pains to explain that this doesn’t mean that two-thirds of cancers are beyond the reach of prevention. But understanding the role of these unforced errors “could provide comfort to the millions of patients who developed cancer but led near-perfect [healthy] lifestyles,” said cancer biologist Dr. Bert Vogelstein of Johns Hopkins University, senior author of both the original study and the new one. “This is particularly true for parents of children who have cancer” and might blame the tragedy on the genes they passed on to their child or the environment they provided, he said.