Leonard Hayflick discovered that cells can replicate themselves about 50 times before they lose their ability to replicate. This is known as the Hayflick limit or Hayflick phenomenon. The Hayflick limit is due to the shortening of telomeres, which are the protective caps on the ends of chromosomes. As cells divide, the telomeres become shorter, and eventually, the cell can no longer divide. The Hayflick limit is an important concept in aging and cancer research, as it helps to explain why cells have a limited lifespan and why cancer cells are able to divide indefinitely.