"Genes with essential functions have been identified in plants through the isolation of gametophytic-lethal and embryonic-lethal mutations. If lethality is a sign of the important metabolic, cellular or developmental function of a given gene, then we may expect some of these genes to also play crucial roles later in plant development. Because homozygotes are not viable or cannot be obtained, the function of lethal genes cannot normally be studied at post-embryonic stages or in the sporophyte. Clonal analysis relies on the ability to induce sectors of clonally-related mutant cells in an otherwise normal individual, opening a window to the study of the effects of lethal mutations in the adult organism. As shown by classical experiments in the model organism Drosophila melanogaster, homozygous sectors for embryo-lethal mutations are often viable (“non-cell lethal”) in the context of normal adult tissues. Hundreds of embryo-lethal and gametophytic-lethal mutants have been isolated in the model plant Arabidopsis thaliana, yet no systematic effort has been undertaken to characterize the functions of the corresponding genes in adult plants. I propose (1) to investigate the post-embryonic functions of lethal genes using clonal analysis techniques, and (2) to generate a system for the induction and marking of twin clones in the model plant Arabidopsis thaliana."