Interleukin-10 (IL-10), also known as cytokine synthesis inhibitory factor (CSIF), is the charter member of the IL-10 family of α-helical cytokines that also includes IL-19, IL-20, IL-22, IL-24, and IL-26/AK155. IL-10 is a 178 amino acid molecule that contains two intrachain disulfide bridges and is expressed as a 36 kDa noncovalently associated homodimer. Mature human IL10 shares 72%86% amino acid sequence identity with bovine, canine, equine, feline, mouse, ovine, porcine, and rat IL10. IL-10, a cytokine produced by CD4+ T lymphocytes belonging to the Th-2 subset, has previously been shown to inhibit the synthesis of IFN-gamma by both T cells and NK cells. The anti-inflammatory actions of IL-10 may be therapeutically useful either directly or through modulation of HO-1 activity.