Use of cell lines to test assumption of linear and symmetrical influence of epithelium and stroma on transcript abundance. Expression data from pure populations of cancer cells grown in monoculture were linearly combined with expression data from pure populations of breast fibroblasts; expected expression values for ratios of epithelium to stroma equal to 3:1, 2:1 or 1:1 were computed. These computed values were compared to observed gene expression for cocultured cells where the actual percentage of epithelial content was 75%, 66% purity, or 50%, respectively. All data for these analyses are available in the Gene Expression Omnibus, GSE22384) and described in Camp et al. . Supplemental Table 2 shows that the correlations between observed and computed were high (0.80-0.92) and that the slopes were very close to 1.