Exhibiting a broad range of biological activities on T and natural killer cells, Interleukin-12 is a disulfide-bonded heterodimeric cytokine composed of a 35-kD IL12A subunit (IL-6 Superfamily) and a 40-kDa IL12B subunit (Type I Cytokine Receptor 3 Family). IL12 stimulates T-cell-independent production of IFN-gamma by PBMCs, enhances NK cell lytic activity, acts as a growth factor for T- and NK-cells, and is important for Th1 and Th2 cell differentiation. Lymphocyte responses to IL12 require NOS2A and are mediated by STAT4 activator of transcription. (NCI)