The lin-12 gene of the nematode Caenorhabditis elegans controls certain binary decisions during development. The lin-12 locus was cloned by means of Tc1 transposon tagging: spontaneous lin-12 null alleles were isolated in a genetic background permissive for Tc1 transposition, and seven independently isolated mutations were found to be associated with Tc1 insertion events. All of these Tc1-induced mutations mapped to a single 2.9 kb restriction fragment within a 50 kb region examined. The DNA sequence of this fragment revealed that, although it does not contain the entire gene, it does include three complete exons. These three exons together encode 11 peptide units that are homologous to one another. The repeated peptide motif is also homologous to a set of mammalian proteins that includes epidermal growth factor.