FADD (Fas Associated via Death Domain) is a critical apoptotic adapter protein that mediates death receptor signaling pathways. Its primary function involves recruiting caspases CASP8 or CASP10 to activated death receptors such as FAS/CD95 or TNFRSF1A/TNFR-1, forming the death-inducing signaling complex (DISC) that enables caspase-8 proteolytic activation and subsequent apoptotic cascades 1. FADD operates through two distinct TNF-α-induced caspase-8 activation pathways: one involving cycloheximide-mediated elimination of c-FLIP inhibitor, and another through RIPK1-FADD-caspase-8 complex formation triggered by Smac mimetics 2. In TNFR1 signaling, FADD participates in a sequential two-complex mechanism where it forms a cytoplasmic complex (complex II) with TRADD, RIP1, and caspase-8, determining cell fate based on NF-κB activation status 3. Beyond apoptosis, FADD exhibits non-apoptotic functions in cellular proliferation and activation, likely regulated by phosphorylation 4. In hepatocellular carcinoma, phosphorylated FADD translocates to the nucleus and upregulates CCL5 transcription via NF-κB, promoting CD8+ T-cell recruitment and enhancing immune checkpoint inhibitor sensitivity 5. Disease relevance includes autoimmune lymphoproliferative syndrome (ALPS) when germline FADD mutations combine with somatic loss of heterozygosity 1, and FIRES (febrile infection-related epilepsy syndrome) caused by homozygous pathogenic variants 6.