Gene expression profile of wild-type MCF-7 cells and long-term estrogen-deprived (LTED) MCF-7:5C breast cancer cells following 17β-estradiol (E2) treatment. Cells were treated with 1 nM E2 for 48 hours, and RNA was hybridized to the Affymetrix Human Genome U133 Plus 2.0 Arrays (Affymetrix, Santa Clara, CA, USA). (a) Hierarchical clustering dendogram of E2-regulated genes in MCF-7 and MCF-7:5C cells. Microarray expression data for each cell line were first filtered for minimal intra-replicate standard deviation (<0.25) and a standard deviation between all samples of at least 0.25. This generated a total of 2,743 genes. In addition, genes displaying a minimum of twofold upregulation or downregulation by E2 in either MCF-7 or MCF-7:5C cells were extracted, revealing a set of 539 differentially expressed, E2-regulated genes. (b) E2 regulation of pro-apoptotic and anti-apoptotic genes in MCF-7 cells (top panel) and MCF-7:5C cells (bottom panel). Bak, Bcl-2 homologous antagonist-killer protein; Bax, Bcl-2-associated X protein; Bcl-2, B-cell lymphoma-2; Bim, Bcl-2-interacting mediator of cell death; GADD45β, growth arrest and DNA damage; p53, 53 kDa protein.