OSM is a 209aa pleiotropic cytokine, originally isolated from PMA (phorbol 12-myristate 13-acetate) treated U937 leukemia cells and mitogen-activated T lymphocytes. It was identified by the ability to inhibit the replication of various human tumor cell lines, but has also been shown to exert a stimulatory effect on fibroblasts, smooth muscle cells and Kaposi's sarcoma cells, and to induce the release of certain cytokines by endothelial cells. OSM signals through binding with low affinity to the gp130 protein present in two cell surface receptors: type I consisting of gp130 and LIFR (leukocyte inhibitory factor receptor) which also acts as a receptor for LIF, and type II consisting of gp130 and OSMR, which mediates OSM specific activities.