Damage to myelin, a membrane sheath that encases axons and speeds up nerve impulse transmission, is linked to certain CNS disorders, such as multiple sclerosis and spinal cord injury. On p. 1443, Su-Chun Zhang and colleagues now report progress in generating oligodendrocytes - the cells that produce myelin in the CNS - from human embryonic stem cells (hESCs),opening up new avenues for both basic and clinical research. Mouse embryonic stem cells (mESCs) can be efficiently differentiated into oligodendrocytes,but this is not the case for hESCs. The authors show that, as with mESCs,treating hESCs with sonic hedgehog induces oligodendrocyte differentiation by triggering the activation of a conserved transcription factor cascade. But in hESCs, this process takes around 14 weeks; in mESCs, it takes just two. In addition, the mitogen FGF2, which promotes oligodendrocyte differentiation in mESCs, stalls it in hESC-derived cultures. Thus, a conserved transcriptional network appears to underlie oligodendrocyte differentiation in human cells,but this network is probably regulated in different ways among species.