Figure 1. Schematic of interleukin-20 receptor complex before and after activation by IL-20, -19, or -24. The type I receptor is composed of the interleukin-20RA (IL-20RA) and IL-20RB proteins while the type II receptor contains the IL-22RA1 and IL-20RB proteins [28,30,61]. Upon cytokine binding, Jak1 binds near the transmembrane region of IL-20RA and IL-22RA1. Tyk2 is proposed to bind to IL-20RB based on sequence homology between IL-20RB and interferon-γ receptor 1 (IFNγR1) [62]. Binding of the cytokine results in closer juxtaposition of Jak1 and Tyk2, leading to autophosphorylation of the tyrosine kinases, phosphorylation of IL-20RA, and recruitment and phosphorylation of signal transducer and activator of transcription (STAT)-3, STAT-1, and possibly, STAT-5 [61,63]. The STATs can then translocate to the nucleus to activate transcription.