Oncostatin M (OSM) was initially identified as a polypeptide cytokine which inhibited the in vitro growth of cells from melanoma and other solid tumors. OSM shows significant similarities in primary amino acid sequence and predicted secondary structure to leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), granulocyte colony-stimulating factor (G-CSF), interleukin 6 (IL-6), and interleukin 11 (IL-11). Analysis of the genes encoding these proteins reveals a shared exon organization suggesting evolutionary descent from a common ancestral gene. Recent data indicates that OSM also shares a number of in vitro activities with other members of this cytokine family. The overlapping biological effects appear to be explained by the sharing of receptor subunits.