Sofia Gruvberger, Markus Ringnér, Yidong Chen, Sujatha Panavally, Lao H. Saal, Åke Borg, Mårten Fernö, Carsten Peterson and Paul S. Meltzer Estrogen Receptor Status in Breast Cancer is Associated with Remarkably Distinct Gene Expression Patterns Cancer Research 61, 5979-5984 (2001) Abstract: To investigate the phenotype associated with estrogen receptor alpha (ER) expression in breast carcinoma, gene expression profiles of 58 node-negative breast carcinomas discordant for ER status were determined using cDNA microarray technology. Using artificial neural networks (ANNs) as well as standard hierarchical clustering techniques the tumors could be classified according to ER status, and a list of genes which discriminate tumors according to ER status was generated. The ANNs could accurately predict ER status even when excluding top discriminator genes including ER itself. By reference to the SAGE database, we found that only a small proportion of the 100 most important ER discriminator genes were also regulated by estradiol (E2) in MCF-7 cells. The results provide evidence that ER+ and ER- tumors display remarkably different gene expression phenotypes not solely explained by differences in estrogen responsiveness. LU TP 01-14