The originally described IL-17 protein, now known as IL-17A, is a homodimer of two 136 amino acid chains, secreted by activated T-cells that act on stromal cells to induce production of proinflammatory and hematopoietic bioactive molecules. Today, IL-17 represents a family of structurally-related cytokines that share a highly conserved C-terminal region but differ from one another in their N-terminal regions and in their distinct biological roles. The six known members of this family, IL-17A through IL-17F, are secreted as homodimers. IL-17A exhibits cross-species bioactivity between human and murine cells. Recombinant human IL-17A is a 31.3 kDa disulfide-linked homodimer of two 137 amino acid polypeptide chains.