A20 (also known as TNFAIP3) is a zinc finger protein and inhibits NF-kappa B activation as well as TNF-mediated apoptosis. The protein has both ubiquitin ligase and deubiquitinase activities, is involved in the cytokine-mediated immune and inflammatory responses. A20 is as a tumor suppressor and has been shown to protect MCF-7 breast carcinoma cells from TNF-induced apoptosis. HighA20expression levels were observed in more aggressive breast tumors (ER-negative, progesterone receptor-negative and high histological grade). These findings strongly suggest that A20 is a key protein involved in tamoxifen resistance.