Oligodendrocyte precursor cells were discovered in 1993 by Raff, Miller and Noble and have been studied extensively. These precursor cells are referred to in the literature as type 2 astrocyte progenitor cells or oligodendrocyte precursor cells (OPCs). Both the developing and adult central nervous systems contain OPCs. Oligodendrocytes, the myelin-forming cells of the central nervous system, develop from OPCs. In culture, OPCs can be generated from neural progenitors or neural stem cells in the presence of basic fibroblast growth factor. They proliferate in the presence of platelet-derived growth factor or factors produced by astrocytes and differentiate into mature oligodendrocytes. Because of these qualities, oligodendrocyte precursor cells are an exceptional population to study developmental transitions.