Redox potential determines whether Sirt1 drives cells towards neuronal or astroglial fates. (a) In an oxidizing environment, Sirt1 forms a complex with Hes1 on the Mash1 promoter, deacetylates histones and recruits repressor proteins such as TLE1. This inhibits transcription of Mash1 and drives the cell towards an astrocyte fate. (b) In a reducing environment, Sirt1 does not bind to Hes1 on the Mash1 promoter. Instead, Hes1 recruits CBP, which activates Mash1 transcription and stimulates the stem cell to assume a neuronal fate.