Interleukin-22 (IL-22) was initially identified as a gene induced by IL-9 in mouse T cells and mast cells. Mouse IL-22 cDNA encodes a 179 amino acid residue protein with a putative 33 amino acid signal peptide that is cleaved to generate a 147 amino acid mature protein that shares approximately 79% and 22% sequence identity with human IL22 and IL10, respectively. IL22 has been shown to activate STAT-1 and STAT-3 in several hepatoma cell lines and up-regulate the production of acute phase proteins. IL-22 is produced by normal mouse T cells upon Con A activation. Mouse IL-22 expression is also induced in various organs upon lipopolysaccharide injection, suggesting that IL-22 may be involved in inflammatory responses. The functional IL-22 receptor complex consists of two receptor subunits, IL-22R (previously an orphan receptor named CRF2-9) and IL-10Rβ (previously known as CRF2-4), belonging to the class II cytokine receptor family.