Placenta growth factor (PlGF) (Maglione et al. 1991, 1992), vascular endothelial growth factor (VEGF) (Ferrara and Henzel 1989; Gospodarowicz et al. 1989; Keck et al. 1989; Levy et al. 1989; Conn et al. 1990), VEGF-B (Olofsson et al. 1996), VEGF-C (Joukov et al. 1996) and Fos-induced growth factor (FIGF) (Orlandini et al. 1996) are members of a family of structurally related growth factors. These factors are all dimeric glycoproteins and share a number of biochemical and functional features (for review, see Bussolino et al. 1997). Intra- and interchain disulphide bonds among eight characteristically spaced cysteine residues are involved in the formation of the dimeric active proteins. PIGF and VEGF can form heterodimeric molecules in cells where both genes are expressed (DiSalvo et al. 1995; Cao et al. 1996a). VEGF, the first member of this family to be isolated, was identified for its ability to stimulate the proliferation of endothelial cells (ECs). VEGF exerts its action through the binding to the two receptors, VEGF receptor-1 (VEGFR-1; also denoted Flt-1) and VEGFR-2 (also denoted Flk-l/KDR), abundantly expressed on the ECs.