fallen 10-fold by 26 hours, and were further reduced by 50 hours. This same decline was also observed with a polyclonal antiserum raised to a different region of the Sir2 protein (data not show). However, this behavior was not a universal feature of proteins in aging cells: Western blotting against the vacuolar protein Vma2 (Figure 6A) and the kinase Pkc1 (data not shown) both showed no decline in aging cells. Therefore, Sir2 protein levels specifically decline in aging cells, however this decline precedes any significant change in LOH rates by 20 hours, or approximately 12 generations.
