Genes of the achaete-scute complex (ASC) participate in the formation of the central nervous system in the Drosophila embryo. Previous genetic analyses have indicated that lethal of scute (l'sc) is the most important gene of the complex in that process. We have obtained antibodies against the l'sc protein to study the expression of the gene during early neurogenesis. The protein is found in groups of embryonic neuroectodermal cells, analogous to the proneural clusters that precede the appearance of precursors of peripheral sensory organs in imaginal epithelia. The groups appear in different regions of the neuroectoderm, accompanying the three successive waves of neuroblast segregation. Most neuroblasts delaminate from these clusters and express position-specific levels of l'sc protein. No significant differences have been found between the distribution of l'sc RNA and protein. Phenotypic analysis of a l'sc deficiency has shown that the gene is required for neuroblast commitment, although this requirement is less widespread than the domain of l'sc expression, suggesting a high degree of redundancy in the function of genes that participate in the process of neuroblast segregation. The ASC genes have been postulated to play a role in the control of NB identity, revealed by the generation of a defined lineage of identifiable neurons. However, our study in l'sc mutants of the expression of fushi tarazu, engrailed, and even-skipped, used as markers of neuronal identity, has not provided evidence to support this hypothesis.