Interleukin 31 (IL-31) is a secreted, T-cell-derived, short-chain member of the alpha-helical family of cytokines. The mature region is 141 amino acids (aa) in length and exhibits four alpha-helices. Human and mouse IL-31 are highly divergent at 24% aa sequence identity. The receptor for IL-31 is found on keratinocytes and monocytes. It is a heterodimer composed of the Oncostatin M receptor and a novel gp130-related molecule. IL-31 may be involved in pruritis, or skin itching.