The National Toxicology Program (NTP) on Thursday released the peer-reviewed partial results of a study into the effects of mobile radiation on rats. It found low incidences of malignant gliomas in the brain, and schwannomas in the hearts of the subjects.

Two forthcoming NTP reports will lay out the full findings of the study and others like it, also involving mice. These reports will probably only be available for peer review and public comment by the end of 2017, although some additional results should be published later this year.

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For now, the researchers say the brain and heart tumors that they observed in some male rats “are considered likely the result of whole-body exposures” to radiofrequency radiation (RFR) modulated using the GSM and CDMA mobile industry standards.

RFR is non-ionizing, which is to say it doesn’t carry enough energy to remove the electrons from atoms or molecules. Ionizing radiation, such as X-rays, comes with known cancer risks. The evidence for RFR being any kind of cancer threat has so far been extremely limited.

Two things are very important to note here. Firstly, rats aren’t people. Secondly, the incidences of tumors in these tests were indeed very low—although with the ubiquity of this technology, even a low risk could be a serious public health concern.

According to former NTP project leader Ron Melnick, quoted by the Wall Street Journal, the new study “ends” the notion that there is no risk from cellphone radiation.

That said, other studies have suggested cellphone radiation is safe. For example, a study in Australia, the results of which were released last month, showed no increase in brain cancer in the country that could be tied to the widespread rise in cellphone use.

The NTP tests involved exposing rodents to the typical radio frequencies used in cellular communications in the U.S. (at 900MHz for the rats and 1900MHz for the mice) for periods of around nine hours per day, starting in the womb and continuing throughout their lives.

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The radiation using the widely-used GSM mobile communications standard resulted in low incidences (up to 3.3%) of malignant brain gliomas in all groups of male rats. Exposure to radio waves using the CDMA standard also caused statistically significant trends towards malignant gliomas (again, at 3.3%) at higher energies. There were no incidences of these lesions in control group rats, which were not exposed to RFR.

Incidences were lower for female rats, with just 1.1% of rats developing a brain lesion from GSM-modulated radiation at higher energies, and 2.2% developing malignant lesions from lower-energy CDMA-modulated radiation.

Heart lesions were again observed in some male rats in all the exposed groups, for both GSM and CDMA, although CDMA radiation caused significantly higher incidences (up to 6.6%). Again, the control group rats saw no incidences, and incidences were lower among females.

As it happens, the control group rats actually had lower survival rates at the end of the two-year study than those exposed to the radiation—the researchers said it was possible this could have had an effect on the comparison between exposed and non-exposed rats, if the tumors in question are late-developing.

“Most of the gliomas were observed in animals that died late in the study, or at the terminal sacrifice,” they wrote. “However, a relatively high number of the heart schwannomas in exposed groups were observed by 90 weeks into the study, a time when approximately 60 of the 90 control male rats remained alive and at risk for developing a tumor.”

The GSMA, which represents mobile operators around the world, stressed that previous studies over the last two decades have not shown any health risks associated with mobile use.

“The study findings will be considered by the scientific community in the context of the whole body of available research, the consistency of the findings and the importance of replication,” a spokesperson told Fortune.