The nuclear receptor coactivator 3 (NCOA3), also known as the steroid receptor coactivator-3 (SRC-3), is a protein that, in humans, is encoded by the NCOA3 gene. NCOA3 is a transcriptional coactivator protein that contains several nuclear receptor interacting domains and an intrinsic histone acetyltransferase activity. NCOA3 acylates histones, which makes downstream DNA more accessible to transcription. Hence, NCOA3 assists nuclear receptors in the upregulation of gene expression. The ratio of PAX2 to AIB-1 protein expression may be predictive of the effectiveness of tamoxifen in breast cancer treatment.