LIF is a multifunctional secreted glycoprotein that exists in both soluble and matrix bound forms. It displays biologic activities ranging from the differentiation of myeloid leukemic cells into macrophage lineage to effects on bone metabolism, inflammation, neural development, embryogenesis, and the maintenance of implantation. It is now clear that LIF is related in both structure and mechanism of action to the interleukin IL6 family of cytokines, which also includes IL11, ciliary neurotrophic factor, oncostatin M, and cardiotrophin 1. The actions of these cytokines are mediated through specific cell-surface receptors that consist of a unique chain and the shared signal transducing subunit gp130.