The p19 protein shares sequence similarity with IL-6 subfamily members and is distantly related to the p35 subunit of IL-12. p19 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. Also similar to IL-12, human IL-23 stimulates IFN-(gamma) 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. The receptors for each appear to share one subunit, but also have at least one distinct subunit. Activated dendritic cells secrete detectable this heterodimeric complex and IL-23 binds to IL-12R beta 1 and IL-23R. At least six spliced isoforms of IL-23R (IL-23R1 to 6) can be generated through alternative splicing. The IL-23R2 and/or IL-23R4 variants are predominantly detected in certain human lung carcinomas and may contribute to their pathogenesis.