Cytokine which acts as a ligand for wgn (PubMed:12894227). Also acts as a ligand for grnd (PubMed:25874673). Induces apoptosis by triggering JNK signaling (PubMed:12176339, PubMed:12065414, PubMed:12894227). Required for JNK-dependent non-autonomous apoptosis through release from apoptotic cells and activation of apoptosis in neighboring cells (PubMed:24066226). Required for JNK-independent damage-induced apoptosis in the embryonic central nervous system through regulation of the pro-apoptotic gene hid (PubMed:25754009). Involved in the innate immune response to extracellular pathogens (PubMed:17381241). Plays a role in the melanization immune response through its involvement in the rupture of crystal cells and subsequent release of prophenoloxidase (PubMed:17356067). Following UV-induced epidermal damage, released from apoptotic epidermal cells, binds to the wgn receptor on nociceptive sensory neurons and plays a role in development of thermal allodynia, a responsiveness to subthreshold thermal stimuli which are not normally perceived as noxious (PubMed:19375319). Involved in glial cell division induced by neuronal programmed cell death and injury (PubMed:19019992). Has tumor suppressor activity and eliminates oncogenic cells from epithelia, thereby maintaining epithelial integrity (PubMed:19289090). During embryogenesis, highly expressed in glial cells and neurons (at protein level) (PubMed:25754009). Highly localized to the dorsal surface of pregastrulating embryos (PubMed:12894227). During gastrulation, expression occurs in the epidermal layer of the embryo and is most prominent at the surface of dorsal folds that will later form the amnioserosa (PubMed:12894227). At germ band extended stages, prominent in the neurogenic region adjacent to the mesodermal segments of the embryo (PubMed:12894227). In later-staged embryos at stages 15/16, detected in subsets of cells within the condensing nerve cord (PubMed:12894227). After embryonic stage 10, predominantly detected in the nervous system (PubMed:12065414). In the third instar larva, strongly expressed in the brain hemispheres and at the morphogenetic furrow in the eye disk (PubMed:12065414). Also expressed at significant levels in many cells posterior to the furrow and in the proliferating cells at the furrow (PubMed:12065414). In the adult, expressed in the fat body (PubMed:20505310). Isoform 1 and isoform 2 are expressed throughout development and in the adult. Isoform 1 predominates at each stage by about 5 to 10-fold. By gamma-radiation (PubMed:12894227). By bacterial infection (PubMed:20505310). N-glycosylated. The soluble form derives from the membrane form by proteolytic processing. Reduced resistance to extracellular pathogens (PubMed:17381241). Complete absence of thermal allodynia in mutant larvae exposed to UV radiation (PubMed:19375319). RNAi-mediated knockdown in the adult fat body results in increased survival following S.typhimurium infection, increased levels of diptericin and reduced feeding rate (PubMed:20505310). Belongs to the tumor necrosis factor family.