We have determined the nucleotide sequence of two transcription units of the AS-C and shown that their potential protein coding regions share two principal domains of homology. Both domains are conserved within the myc protein family, and one of them is highly homologous with the consensus for protein tyrosine kinase substrates. We show in addition that at least one of these domains is shared with other transcription units within the AS-C, some of which were not previously known. We suggest that the AS-C encodes several homologous polypeptides, which represent a subset of a larger gene family. We propose that each member of the family functions at an equivalent stage of a unique morphogenetic operation involving the segregation of individual neural lineages from the epidermal anlage.