The Interleukin-12 heterodimer (IL-12) is formed by Interleukin-12 subunit alpha (IL12A, IL-12p35) and Interleukin-12 subunit beta (IL12B, IL-12p40) (Podlaski et al. 1992, Kobayashi et al. 1989, Stern et al.1990). Heterodimerization occurs due to charged residue interactions (Yoon et al. 2000) and is stabilized by disulfide bounding. IL12 heterodimerization is believed to occur in the endoplasmic reticulum (ER) because inhibition of the enzyme Prolyl 4-hydroxylase subunit beta (P4HB), referred to in this article as Protein disulfide isomerase (PDI), by bacitracin causes IL-12 retention in the ER (Alloza & Vandenbroek 2005).