Even the simplest animal nervous system contains numerous cell types. In Drosophila, such diversity arises through neural precursors (called neuroblasts, NBs) dividing asymmetrically and generating a stereotyped sequence of neuronal and glial progeny. This process is controlled by a set of sequentially expressed regulators, the temporal identity factors, which specify a neuron's fate, depending on when it was `born' during neurogenesis. In addition, timing factors, such as Seven up, define how long each temporal identity factor is expressed and thus the number of each type of neuron produced. Now, Tran and Doe use a newly characterized NB lineage to show that two late temporal identity factors, Pdm and Castor, also function as timing factors, while Hunchback and Kruppel act as early temporal identity factors,as in other previously studied lineages (see ). These findings highlight the importance of lineage-specific cues in modulating the function of temporal identity and timing genes. For more on the temporal control of neuronal diversity, see the Hypothesis by Gould and colleagues on .
