The four types PDGFs and their domain compositions. The starting numbers of specific domains/segments in the coding sequences are marked above the boundaries. Shown are all numbers for human PDGFs. The proprotein convertase-recognition sequences are indicated and positions of cleavage are marked with a black triangle. Abbreviations used: SP: signal peptide; PRO: the pro-sequence preceding the growth factor domain; Cys-Knot, the cysteine-knot growth factor domain that is responsible for receptor recognition; CUB, the Complement subcomponents C1r/C1s, Urchin EGF-like protein, and Bone marrow protein 1-like domain, which can be considered as part of the pro-sequence in PDGF-C and PDGF-D, but PDGF-C and PDGF-D also have sequences homologous to the PDGF-A and PDGF-B pro-sequences.