Although some lines of evidence suggest that neurogenesis in the adult hippocampus involves neural stem cells (NSCs) that can differentiate into oligodendrocytes, neurons and astrocytes, fate-mapping studies suggest that only astrocytes and neurons are generated. Here, conditional inactivation of neurofibromin (NF1) resulted in the generation of a large number of oligodendrocyte precursor cells (OPCs) that were not present in controls, suggesting that NF1 actively suppresses the OPC-producing potential of NSCs in the mature hippocampus.