After dimerization of the PDGF-A and PDGF-B chains in the ER of producing cells, the dimers are proteolytically cleaved in the trans-Golgi network during protein maturation and secretion. The dibasic-specific proprotein convertase, furin, or related convertases are involved in the conversion of proPDGF forms to active PDGF forms. PDGF-A chains are expressed as two different isoforms, a longer and a shorter form.The longer (241 aa) is less common and differs from the shorter one (196 aa) by a C-terminal extension of 18 aa (Beckmann et al. 1988, Ostman et al. 1992). The PDGF-A chains are cleaved singly at the RRKR sequence at 86 position to yield predominantly, the secreted PDGF-AA forms, while PDGF-BB are reported that at least three forms of PDGF-BB can be formed (Seidah & Prat 2002). This includes an approx 24 kDa form retained intracellularly and degraded in lysosomes, a secreted approx 30 kDa form and an approx 40 kDa cell surface-associated form. PDGF-B is processed at the 'RGRR' sequence at position 81 and a second clevage close to residues 'ARPVT' at position 190 (Siegfried et al. 2005, Ostman et al. 1992, Heldin & Westermark 1999).