Subunit beta of interleukin 12 （also known as IL-12B, natural killer cell stimulatory factor 2, cytotoxic lymphocyte maturation factor p40, or interleukin-12 subunit p40） is a protein subunit that in humans is encoded by the IL12B gene. IL-12B is a common subunit of interleukin 12 and interleukin 23. This cytokine is expressed by activated macrophages that serve as an essential inducer of Th1 cells development. This cytokine has been found to be important for sustaining a sufficient number of memory/effector Th1 cells to mediate long-term protection to an intracellular pathogen.