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.