There has been longstanding interest in understanding how the process by which progenitors differentiate into different cell types is regulated. In a mouse model, the fate of progenitors in the developing brain is believed to be determined by external cues that involve various types of cytokines and internal cellular programs. External cues such as bone morphogenetic proteins, leukemia inhibitory factor, ciliary neurotrophic factor, Notch-Delta, and basic fibroblast growth factor promote astrocytic differentiation [7] [8] [9] [10] [11] [12] [13] , and most of these factors influence the essential astrogliogenic Janus kinase-signal transducer and activator of transcription pathway [14] [15] [16] [17] . A molecular basis for the cooperative action between these families of cytokines involves the formation of a STAT3-Smad1 complex with the coactivator, p300/CBP, that initiates astrocytespecific gene expression [15, [18] [19] [20] .