An important developmental process is the switch from proliferation to terminal differentiation that occurs in the precursor cells of many cell lineages. Billon and co-workers study this process in cultures of oligodendrocyte precursor cells (OPCs) isolated from the rat optic nerve. On p. 1211, they report that the transcription factors p53 and p73, but not the closely related p63,have important roles in promoting OPC differentiation. OPCs can be stimulated to proliferate in serum-free culture by platelet-derived growth factor (PDGF);withdrawal of PDGF or addition of thyroid hormone induces OPC differentiation. In the current study, the researchers used retroviral vectors to overexpress p53, p63 and p73, and dominant-negative versions of these transcription factors in OPCs. Their results indicate that p73 is needed to induce OPC differentiation in response to PDGF withdrawal and that both p53 and p73, but not p63, are needed for thyroid hormone-induced differentiation.