Metformin protects Fancd2^−/− mice from tumor development. Kaplan-Meier survival curves of the Fancd2^−/−Trp53^+/− mice and Fancd2^+/+Trp53^+/− mice. For Fancd2^−/−Trp53^+/− mice, the data represent 31 mice for metformin treatment and 60 mice for placebo treatment. For Fancd2^+/+Trp53^+/− mice, the data represent 30 mice for metformin treatment and 60 mice for placebo treatment. Tumor samples and selected tissues were fixed in 10% phosphate-buffered formalin, stained with hematoxylin and eosin, and examined under a microscope. The Kaplan-Meier survival curves were generated by Prism 6.0c Software (GraphPad Software, Inc.), and P values were calculated using the log-rank (Mantel-Cox) test.