Platelet-derived growth factor (PDGF) [1,2] is a potent mitogen for cells of mesenchymal origin, including smooth muscle cells and glial cells. PDGF consists of two different but closely related chains (A and B chains) which assemble to form disulfide linked homo- or heterodimers (A-A, B-B, an A-B). Alternate splicing of the A chain transcript can give rise to two different forms that differ only in their C-terminal extremity. The transforming protein of simian sarcoma virus (SSV), encoded by the v-sis oncogene, is derived from the B chain of PDGF.

PDGF is structurally related to a number of other growth factors which also form disulfide-linked homo- or heterodimers:

• Vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF) [3], a growth factor active in angiogenesis and endothelial cell growth. The genome of the orf poxvirus encodes an homolog of VEGF [4].
• Vascular endothelial growth factor B (VEGF-B), also active in angiogenesis and endothelial cell growth [5].
• Vascular endothelial growth factor B (VEGF-C), also active in angiogenesis and endothelial cell growth [6].
• Placenta growth factor (PlGF) [7], which is also active in agiogenesis.

As a signature pattern for this family of growth factors, we selected a region that include four of the eight cysteines conserved in the sequences of these proteins. In PDGF, these cysteines are known to be involved in intra- and inter-chain disulfide bonds [8]. We also developed a profile that spans the eight conserved cysteines.