Lifespan: The Revolutionary Science of Why We Age—and Why We Don't Have To

by David A. Sinclair

In other words, the cell doesn’t make enough Sir2 protein to simultaneously silence the mating-type genes and repair broken DNA; it has to shuttle Sir2 between the various places on an “as-needed” basis. This is why adding an extra copy of the SIR2 gene extends lifespan and delays infertility:

Adding extra copy of Sir2