Using Chromatin Immunoprecipitation assay with sequencing (ChIP-seq) analyses, it was observed that ARID1A binding decreases with estrogen and increases after 4-hydroxytamoxifen treatment, and its loss led to upregulation in the expression of tamoxifen-response genes, suggesting ARID1A as a corepressor and tumor suppressor. This evidence concerns the gene ARID1A and neoplasm.