ESR1 and breast cancer: By recreating gene modules of two prominent biological signals present in breast cancer, ER and HER2, we have demonstrated that if the gene-modules derived from FFPE samples represents an incompletely recovered biological signal, such as the ER/HER2 de-novo modules extracted from u133p2.3ivt.ffpe, it is unlikely to be reproducible in external datasets (Figs 4 and 5).