The genetic control of cell lineage has been studied extensively in Caenorhabditis elegans. In this paper, three studies of cell lineage mutants are reviewed: the isolation of mutations affecting vulval cell lineages, and the analysis of two 'control genes',
lin-12 and
lin-14. In addition, certain logical features of the genetic programme, as inferred from or illuminated by the study of cell lineage mutants, are discussed: the concepts of 'control genes' and developmental subprogrammes, and the organization of the lineage into a hierarchy of binary