Acts as a negative mediator of apoptosis and may play a role in tumor progression. Suppresses the TNF-mediated apoptosis by inhibiting caspase-8 activity but not the processing of procaspase-8, subsequently resulting in inhibition of BID cleavage and caspase-3 activation. Expressed at high levels in the spleen, lymph node, thymus, thyroid, bone marrow and placenta. Expressed at high levels both in various tumor tissues, unstimulated and cytokine-activated cultured cells. Expressed at low levels in the spinal cord, ovary, lung, adrenal glands, heart, brain, testis and skeletal muscle. Expressed at high levels in the fetal liver, lung and kidney. By nuclear factor-KB (NF-KB) and TNF. Induction by TNF depends upon activation of NF-KB. Belongs to the TNFAIP8 family.