Tamoxifen is effective for treating estrogen receptor-alpha (ER) positive breast cancers. However, few molecular mediators of tamoxifen resistance have been elucidated. Here we describe a previously unidentified gene, MACROD2 that confers tamoxifen resistance and estrogen independent growth. We found MACROD2 is amplified and overexpressed in metastatic tamoxifen-resistant tumors. Transgene overexpression of MACROD2 in breast cancer cell lines results in tamoxifen resistance, whereas RNAi-mediated gene knock down reverses this phenotype. MACROD2 overexpression also leads to estrogen independent growth in xenograft assays. Mechanistically, MACROD2 increases p300 binding to estrogen response elements in a subset of ER regulated genes. Primary breast cancers and matched metastases demonstrate MACROD2 expression can change with disease evolution, and increased expression and amplification of MACROD2 in primary tumors is associated with worse overall survival. These studies establish MACROD2 as a key mediator of estrogen independent growth and tamoxifen resistance, as well as a potential novel target for diagnostics and therapy.breast cancer | tamoxifen | resistance | MACROD2 | ER positive T he selective estrogen receptor modulator (SERM) tamoxifen is a highly effective drug for the prevention and treatment of estrogen receptor-alpha (ER) positive breast cancers (1). However, resistance to this drug remains a clinically important problem. The molecular mediators of tamoxifen resistance have not been fully elucidated. In part, this is due to the heterogeneous nature of breast cancers, resulting in multiple mechanisms of resistance. For example, past studies have demonstrated that tamoxifen resistance is mediated by differential expression of nuclear hormone receptor coregulators (2, 3), growth factor signaling crosstalk (4-7), regulation of microRNAs (8), cyclin dependent kinases (CKDs) (9), CDK inhibitors (10, 11), and more recently, acquired somatic mutations and alterations in ER (12-17). Further insight into the molecular mediators of tamoxifen and hormone therapy resistance would have great impact on the ability to target genes and pathways that could overcome drug resistance and lead to improved clinical outcomes.In this study we describe a previously unidentified gene, MACROD2, which is amplified and overexpressed in a subset of breast cancers. MACROD2 belongs to a family of genes containing a macro domain, an evolutionarily conserved protein motif (18), whose functional role until recently has been unclear. Studies have demonstrated that MACROD2 deacetylates O-acetyl-ADP ribose, a signaling molecule generated by the deacetylation of acetylated lysine residues in histones and other proteins (19). More recent work demonstrates that MACRO domain containing proteins are involved with mono-ADP ribosylation, and can regulate cell signaling pathways and modify proteins involved with gene transcription (20). Interestingly, MACROD1 (LRP16) has been implicated in modulating ER and androgen receptor (AR) signaling in prio...