ESR1 and breast cancer: Microarray-based mRNA and miRNA expression profiles of tamoxifen-resistant (TR) and tamoxifen-sensitive (TS) ER+ breast cancer patients were extracted from relevant datasets available at the Gene Expression Omnibus (GEO), followed by DEMs analyses and DEGs analyses, prediction of downstream target genes, and subsequently gene regulatory network to determine potential biomarkers associated with tamoxifen resistance.