IL-10, or cytokine synthesis inhibitory factor (CSIF), is the founding member of the IL-10 family of α-helical cytokines, which also includes IL-19, IL-20, IL-22, IL-24, and IL-26/AK155. This 178-amino acid molecule, with two intrachain disulfide bridges, is expressed as a 36 kDa noncovalently associated homodimer. Human IL-10 shares 72-86% amino acid sequence identity with various species. Produced by CD4+ T lymphocytes of the Th-2 subset, IL-10 inhibits IFN-gamma synthesis by T cells and NK cells. Its anti-inflammatory actions may be therapeutically beneficial, either directly or by modulating HO-1 activity.