Figure 1: The specification of the spinal cord neurons directed by a combinatorial code of transcription factors. Schematic summary of the expression of bHLH transcription factors in the progenitor cells (left) and homeodomain (HD) transcription factors in the differentiating and differentiated neurons. Eleven early classes of postmitotic neurons (dI1~6, V0~3, and motoneurons (MN)) and two late-born dorsal interneurons (dILA and dILB) are present in the embryonic spinal cord. The immature postmitotic V0 and V2 interneurons are further subdivided into two distinct interneuron subtypes indicated by arrows. The glutamatergic excitatory neurons and GABAergic inhibitory neurons are represented by red and blue circles, respectively.