Migration stimulating factor (MSF) is a 70 kDa truncated isoform of fibronectin containing a unique intron-derived 10 amino acid C-terminal sequence not present in any previously described full-length (250 to 280 kDa) fibronectin isoform. Unlike fibronectin, MSF is not an extracellular matrix component, but a soluble factor showing potent bioactivities relevant to cancer development, such as stimulation of cell migration, matrix remodelling and angiogenesis. MSF motogenic activity is mediated by its constituent IGD (isoleucine–glycine–aspartate) motifs. Two isoforms of MSF have been cloned. These differ by a 15 amino acid deletion and are referred to as MSF+aa (AJ535086) and MSF-aa (AJ276395). The term MSF will be used to denote both isoforms. We have identified an inhibitor of MSF+aa (MSFI) that is present in serum and in approximately 50% to 65% of breast carcinomas. MSF+aa and MSF-aa differ in their functional interaction with this inhibitor: MSF+aa is inhibited by MSFI, whereas MSF-aa is not. Both isoforms contain the same MSF-specific sequence and IGD functional domains; consequently, they are equally active in the absence of MSFI [1–4] (K. Kankova, S.J. Jones, I.R. Ellis, M.M. Florence, S.L. Schor, A.M. Schor, unpublished data, 2008).