The p19 shares sequence similarity with IL-6 subfamily members and is distantly related to the p35 subunit of IL-12. It shows no biological activity by itself, instead, it combines with the p40 subunit of IL-12 to form a biologically active, composite cytokine, IL-23. IL-23 shares some in vivo functions with IL-12, including the activation of the transcription factor Stat4. Similar to IL-12, human IL-23 stimulates IFN-g production and proliferation in PHA blast T cells, as well as in CD45RO (memory) T cells. Ubiquitous transgenic expression of the IL-23 subunit p19 induces multiorgan inflammation, runting, infertility, and premature death.Synonyms: IL-23 subunit alpha, IL-23p19, Interleukin-23 subunit p19, SGRF