Leukemia Inhibitory Factor (LIF) is a lymphoid factor that promotes long-term maintenance of embryonic stem cells by suppressing spontaneous differentiation. LIF has a number of other activities including cholinergic neuron differentiation, control of stem cell pluripotency, bone and fat metabolism, mitogenesis of certain factor dependent cell lines and promotion of megakaryocyte production in vivo. Human and murine mature LIF exhibit a 78% sequence identity at the amino acid level. Human LIF is equally active on human and mouse cells. Murine LIF is approximately 1000 fold less active on human cells than human LIF.