Upd3--an ancestor of the four-helix bundle cytokines.

The unpaired-like protein 3 (Upd3) is one of the three cytokines of Drosophila melanogaster supposed to activate the JAK/STAT signaling pathway (Janus tyrosine kinases/signal transducer and activator of transcription). This activation occurs via the type-I cytokine receptor domeless, an orthologue of gp130, the common signal transducer of all four-helix bundle interleukin-6 (IL-6) type cytokines. Both receptors are known to exist as preformed dimers in the plasma membrane and initiate the acute-phase response.