Molecular evidence for the temporal diversification of Drosophila NB progeny comes from studies of the zinc-finger transcription factor Castor (Cas). An analysis of Cas transcriptional regulatory targets revealed that the DNA-binding specificity of Cas is similar if not identical to that of another structurally different zinc-finger protein known as Hunchback (Hb). Previous work examining Hb transcriptional targets identified two genes encoding the functionally redundant POU domain transcription factors, pdm-1 and pdm-2, referred to here as pdm. In the cellular blastoderm, Hb is a repressor of pdm expression. It was reasoned that Hb might target pdm during neurogenesis, and as Cas binds to Hb consensus binding sites, perhaps Cas too regulates pdm during neurogenesis. Subsequent studies have shown that Hb and Cas act early and late, respectively, to restrict pdm expression to a subset of neural cells that maintain an intermediate position sandwiched between early-born neural progeny expressing Hb and late-born neural progeny expressing Cas. Additional lineage marking studies and in vitro culture studies have revealed that these layered expression domains are formed by transitions in NB gene expression, and that the GMC and neural progeny maintain expression of the transcription factor active in the NB during each temporal window. Taken together, these studies indicate that many NBs undergo sequential changes in their gene expression profiles during lineage development (Brody and Odenwald, 2002).