Three DNA repair systems (photoreactivation, excision repair and post-replication repair) exist in most organisms. Their status has been examined in the nematode C. elegans. This metazoan is deficient in photoreactivation but possesses efficient excision and post-replication repair systems. The stage-specific variations in hypersensitivity displayed by radiation-sensitive (rad) mutants, as well as the stage-specific excision-repair deficiency displayed by
rad-3, indicate that DNA repair is developmentally regulated in this popular model system. In addition, various data suggest that C. elegans may possess novel