Cells with the phenotypic characteristics of oligodendrocyte progenitors (NG2+/PDGF alpha R+/O4+) are found throughout the adult mammalian CNS in numbers similar to microglia. They are a reactive glial cell population and respond to demyelination by increasing in number, thereby repopulating the lesion site with cells capable of differentiating into remyelinating oligodendrocytes. Direct evidence that they differentiate into remyelinating cells is missing, although this is the most likely scenario. Cells with the same phenotype are found in normal human CNS tissue and also in chronic MS lesions. Further studies on this intriguing cell type are necessary in order to understand the molecular signals involved in their reaction to injury, particularly in multiple sclerosis.