The sequential appearance of neurons and glia in the vertebrate central nervous system may be governed by competition between growth factor signaling pathways and downstream transcription factors. In cortical progenitor cell cultures, the proneural basic helix-loop-helix transcription factor Ngn1 suppresses formation of astrocytes by sequestering coactivator proteins that are required by signal transducers and activators of transcription for the expression of astrocyte-specific genes. In the developing neural tube, combinatorial interactions between the proneural transcription factor Ngn2 and the basic helix-loop-helix transcription factor Olig2 specify the formation of motor neurons or oligodendrocytes.