“Given the widespread global usage of mobile communications among users of all ages, even a very small increase in the incidence of disease resulting from exposure to [radio-frequency radiation] could have broad implications for public health,” the NTP said. The $25 million study, overseen by the National Institutes of Health, used rats and mice, exposing them to radio frequencies from GSM and CDMA devices, the two most common types of consumer wireless technologies. Only the male rats appeared to experience a boost in cancer rates.

Experiments on rodents and other lab animals don’t always translate to humans, and a number of other studies have found no link between cancer and cellphones, including a recent study from Australia that found no rise in brain cancer since cellphones were introduced in the 1980s. But “where people were saying there’s no risk, I think this ends that kind of statement,” Ron Melnick, who ran the NTP project until retiring in 2009, told The Wall Street Journal. The full study, slated to be released by the fall of 2017, could prompt the U.S. government to modify its safety guidelines, including recommending you talk only with a headset or avoid carrying your phone in your pocket.