FAIM (Fas apoptosis inhibitory molecule) was identified as a protein that was inducibly expressed in B lymphocytes resistant to Fas-mediated apoptosis (1). Expression of FAIM inhibits receptor-mediated apoptosis in B cells as well as other cell types (1-3). FAIM is expressed in germinal center B cells, is positively regulated by IRF-4, and is also capable of inducing IRF-4 expression in a feed-forward mechanism (4). FAIM also regulates T cell receptor-mediated apoptosis by modulating Akt activation and Nur77 expression (2). Knockout mice for FAIM show an increased sensitivity to Fas-mediated apoptosis within B and T cells as well as hepatocytes (5). An alternatively spliced form of FAIM, termed FAIM-L, is found predominantly in the brain (6). In the nervous system, the originally identified FAIM does not appear to play a role in apoptosis, but rather can promote neurite outgrowth through the activation of Erk and NF-κB pathways (7). In contrast, FAIM-L does inhibit neuronal cell death triggered by death receptors (3).
