Fas-associated factor-1 (FAF1) is a Fas-binding pro-apoptotic protein that is a component of the death-inducing signaling complex in Fas-mediated apoptosis. FAF1 is involved in negative regulation of NF-kappaB activation. In addition to the previously known function as a component of the Fas death-inducing signaling complex, i.e. NF-kappaB activity suppressor by cytoplasmic retention of NF-kappaB p65 via physical interaction. hFAF1 was expressed abundantly in testis, skeletal muscle, and heart as 2.8 kb mRNA. Polyclonal antibody against hFAF1 detected 74 kD protein, a deduced protein size from the ORF and 40 kD protein in some cell lines.