**2.6 Statistical analysis**

The significance of differences between tumor multiplicity data sets was determined by the Wilcoxon rank-sum test, using Mstat software (version 5.4, McArdle Laboratory for Cancer Research; URL, http://www.mcardle.wisc.edu/mstat). All *P-*values were calculated on the basis of a two-sided test, except in the case of spontaneous tumorigenesis, for which we were testing the one-sided hypothesis that C3H alleles that increase DEN-induced tumor multiplicity also increase spontaneous tumor multiplicity. Differences between RNA expression data sets were evaluated with the limma software package (Smyth, 2004), using a moderated t-statistic followed by FDR correction.
