The IL-12/IL-23 p40 monomer is a subunit shared between the interleukin-12 （IL-12） and interleukin-23 （IL-23） cytokines. It is a critical component of both the IL-12 and IL-23 heterodimers, which are proinflammatory cytokines that play essential roles in regulating immune responses. The IL-12/IL-23 p40 monomer is involved in the formation of the biologically active IL-12 and IL-23 heterodimers, which bind to their respective receptors and activate signaling pathways that modulate immune cell function. Notably, the p40 subunit itself has been shown to exhibit biological activity independent of its role in forming the heterodimers, including the ability to act as an antagonist for IL-12 and IL-23 signaling by competing for receptor binding.