We have determined the nucleotide sequence of two genes of the achaete - scute complex (AS-C) and show that they are homologous to two previously sequenced members of the same locus. These four genes are interspersed with other transcription units of unknown function. We also study the expression of one of these genes by in situ hybridization and compare it with the other three genes. We suggest that the complete function ascribed to the AS-C by genetic experiments is carried out by the four homologous genes. We discuss the possible biochemical function of the AS-C in neurogenesis in the light of the homologies of the four genes with the mammalian myc family.