Platelet-derived growth factor A (PDGF-A) is a widely expressed cytokine that can form disulfide-linked homodimers or heterodimers with PDGF-B. These proteins regulate diverse cellular functions by binding to and inducing the homo- or hetero-dimerization of PDGF R alpha and R beta. PDGF-AA binds to alpha receptor homodimers, while PDGF-AB additionally binds to the heterodimeric receptor. PDGF-AA and -AB promote cell proliferation, cell survival, cytoskeletal rearrangement, cell migration and extracellular matrix production. They are important for the development of oligodendrocytes, testicular Leydig cells, alveolar smooth muscle cells, hair follicles, and intestinal villus.