According to the United States' National Toxicology Program, malignant tumors have been found in the brains and hearts of male rats. 

Two thousand and five hundred rats were exposed to frequencies and modulations used in cellular communications for 9 hours a day over two years. In 2011, The World Health Organization classified such radiation as a possible carcinogen.  

But previous human, observational data collected in large-scale, population-based studies have found limited evidence of an increased risk of developing cancer from cell phone use.