Interleukin-12 is a disulfide-linked heterodimeric protein (p70), composed of two subunits, p35 and p40, which are encoded by two different genes. Accumulating data indicate that p40 secretion precedes that of IL-12 expression. In addition to its ability to covalently bind to p35 to form IL-12, p40 can bind to p19 to form IL-23. Recombinant human IL-12 p40 is a 40 kDa protein containing 306 amino acid residues.