Fig. 7. Hb and Kr specify early temporal identity, whereas Svp, Pdm and Cas act as timer elements within the NB3-1 lineage of the Drosophila CNS. Spatial cues specify NB3-1 identity and formation, which allows the neuroblast to respond in a potentially unique way to timer genes and temporal identity genes that are expressed subsequently. Timer elements (top row) include Seven up (Svp), Pdm and Cas. These factors close successive temporal identity windows. Timer genes indirectly control cell fate through the regulation of temporal identity genes. Temporal identity genes (middle row) include Hb and Kr, which specify first and second temporal identities (TIs), respectively, in this and other lineages. Neuronal identity (bottom row) within the lineage: during the first TI, GMC-1 makes RP1/sibling neurons and GMC-2 makes RP4/sibling neurons; during the second TI, GMC-3 makes RP3/sibling neurons; during the third TI, GMC-4 makes RP5/sibling neurons; and during the fourth TI, GMC-5 makes interneurons (IN).