IL-12 is a disulfide-linked heterodimeric protein (p70), composed of two subunits, p35 and p40, which are encoded by two different genes.? Accumulating data indicates 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 40.0 kDa protein containing 306 amino acid residues.