Fas apoptosis inhibitory molecule 1 (FAIM)

Fas apoptosis inhibitory molecule 1 (FAIM, also known as FAIM-S/FAIM-L) is a highly conserved, ubiquitously expressed 20 kDa cytosolic protein that confers resistance to Fas receptor-mediated apoptosis primarily through inhibition of caspase activation and stabilization of anti-apoptotic factors[1][2][4]. The gene encodes two main isoforms: **FAIM-S** (short), present in all tissues, and **FAIM-L** (long), specifically abundant in neurons where it promotes neuronal survival, plasticity, and axonal outgrowth[1][2][4]. FAIM1 not only inhibits cell death triggered by Fas ligand and TNFα signaling but also plays a role in preventing the aggregation of amyloid and other pathogenic proteins, implicating it in neurodegenerative disease mechanisms, including Alzheimer's disease[2][4]. Unlike the related but distinct proteins FAIM2 (Lifeguard) and FAIM3 (TOSO), FAIM1 plays no direct role as a death receptor or classical signaling enzyme, but acts as a soluble regulator of cell survival and protein homeostasis[2][4]. Low FAIM1 expression is associated with increased neuronal vulnerability, while dysregulated apoptosis inhibition may also be relevant in cancer biology[2][4]. **Note:** The term "Fas apoptotic inhibitory molecule" also sometimes refers to FAIM2 (Lifeguard, LGI) and FAIM3 (TOSO), but these are distinct proteins with different structure and functions. This entry focuses on FAIM1 as the canonical molecule typically referenced by "Fas apoptotic inhibitory molecule."[2][3][4]

Other names

FAIMFas apoptosis inhibitory molecule short (FAIM-S)Fas apoptosis inhibitory molecule long (FAIM-L, brain-specific isoform)Fas apoptotic inhibitory molecule 1

Drugs or molecules affecting **Fas signaling** or **apoptosis** can influence FAIM1’s protective effects by: - Inhibiting Fas receptor activation - Modulating downstream caspases - Stabilizing anti-apoptotic proteins (e.g., X-linked inhibitor of apoptosis protein (XIAP)) - Antagonistic action on protein aggregation (relevant for neurodegenerative mechanisms)