“…Nonsense suppressors are defined as tRNAs that insert amino acids at positions in a message containing a translational stop codon. Nonsense suppressor tRNA genes have been extensively studied in bacteria [Celis and Smith, 1979;Steege and SOH, 19791 but also in the eukaryotes Saccharomyces cerevisiae [Sherman, 19821 and Schizzosaccharomyces pombe [Munz et al, 1983;Hottinger et al, 19821. Nonsense suppressors were isolated also from Caenorhabditis elegans [Wills et al, 1983;Hodgkin, 1985;Fire, 19861, and recently suppressor tRNA genes were expressed in Drosophila melanogaster [Doerig et al, 19881 and in human cell lines [Laski et al, 1982;Temple et al, 1982;Laski et al, 1984;Capone et al, 19851. Certainly for Escherichia coli and yeast the availability of a significant collection of defined suppressors was crucial for the rapid development of genetics in these systems, and nonsense suppression as a general conditional-lethal system will be of great value for higher eukaryotes as well.…”