Metastasis-associated protein 3 (MTA3) is a subunit of the Mi-2/NuRD transcriptional corepressor complex. MTA3 and the Mi-2/NuRD complex mediate repression of Snail in breast cancer cells where MTA3 works to maintain a differentiated, epithelial status. The protein is involved in gene expression regulation by covalent modifications of histone proteins. There are two known isoforms of MTA3, a short and a long form. The short isoform binds to ER and sequesters it to the cytoplasm and betters non-genomic responses, whereas the long form is found in the nucleus. MTA3 is widely expressed in with highest expression in brain, adrenal glands, ovaries and virgin mammary glands. It has been found to be expressed in higher levels in tumors than in adjacent normal tissue in the same individual.