The approximately 300 distinct neurons comprising each hemineuromere of the 
Drosophila embryonic central nervous system are derived from a segmentally 
reiterated array of approximately 30 progenitor cells, neuroblasts (NBs). Each 
NB has a unique identity and undergoes repeated cell divisions to produce 
several smaller secondary precursor cells, ganglion mother cells (GMCs); each 
GMC divides once to produce two neurons and/or glia, thereby generating a 
specific lineage of neurons/glia. Understanding the generation of neuronal 
diversity requires not only elucidation of the molecules and mechanisms that 
specify NB identity but also those that act to differentiate between the cell 
types produced within one NB lineage. Here we show that the Drosophila Zn finger 
protein Klumpfuss (Klu), which shows sequence similarities to the mammalian 
Wilm's tumor suppressor (WT-1), acts to differentiate between the identities of 
the first two secondary precursor cells produced from one NB lineage. Klu is 
expressed in the NB4-2 lineage only after two rounds of NB cell division, in the 
second born GMC (GMC4-2b). In loss-of-function mutant embryos, the first born 
GMC (GMC4-2a) as well as its progeny neurons are duplicated; we show that this 
duplication of the GMC4-2a sublineage arises because GMC4-2b adopts the identity 
of GMC4-2a and divides to produce the GMC4-2a progeny. Moreover, when Klu is 
ectopically expressed in GMC4-2a, it fails to acquire its normal identity and 
fails to produce correctly specified progeny. klu therefore acts to specify the 
identity of GMC4-2b and to make it distinct from GMC4-2a. Our findings further 
suggest that the determination of GMC cell fate occurs in two steps; the initial 
GMC identity is the consequence of inheritance from the maternal NB, however, 
the subsequent stabilization of this identity requires functions like klu in the 
GMC.