Platelet-derived growth factor-D (PDGF-D) is a cytokine that associates into disulfide-linked homodimers that bind to PDGF R beta and trigger signaling through PDGF R beta homodimers and PDGF R alpha/beta heterodimers. It is secreted as a latent homodimer which is activated by proteolysis to release a bioactive disulfide-linked dimer. PDGF-DD is widely expressed in embryonic and adult tissues. It functions as a growth factor for renal artery smooth muscle cells and lens epithelial cells, and as a macrophage chemoattractant. PDGF-DD is upregulated in and contributes to several disease states, including renal and hepatic fibrosis, mesangial proliferative glomerulopathy, pulmonary lymphoid infiltration, and many cancers.