Leukemia inhibitory factor, an interleukin 6 class cytokine, is a protein in cells that affects cell growth and development. LIF derives its name from its ability to induce the terminal differentiation of myeloid leukaemic cells. Other properties attributed to the cytokine include: the growth promotion and cell differentiation of different types of target cells, influence on bone metabolism, cachexia, neural development, embryogenesis and inflammation.

LIF is normally expressed in the trophectoderm of the developing embryo, with its receptor LIFR expressed throughout the inner cell mass. As embryonic stem cells are derived from the inner cell mass at the blastocyst stage, removing them from the inner cell mass also removes their source of LIF.