() Average life spans for FY833 (WT), ID-1, ID-2 and ID-8 strains were 34.1, 31.9, 20.6 and 22.7 generations, respectively. () Average life spans for the mutants, ID-10, ID-11, ID-12 and ID-13 were 42.5, 41.2, 30.1 and 19.9 generations, respectively. () Detection of the ERCs. DNA isolated from various strains was subjected to electrophoresis (0.6% agarose gel for 30 h at 1 V/cm) followed by Southern hybridization using 5S rDNA as a probe. Thin arrows represent ERCs and thick arrow indicates hybridization signal of the genomic rDNA copies. M; Lambda DNA/HindIII marker.