The maximum number of cell divisions that will take place in human cells prior to their death. In 1961 Hayflick and P. S. Moorehead showed that human cells can reproduce themselves a finite number of times. This limited replication ability is postulated to correlate with the aging, failure, and eventual death of organs and individuals. Hayflick’s limit is exceeded by some cell lines that exist solely to reproduce themselves, such as blood-forming cells and cancer cells. It can also be extended artificially by manipulation of a cellular enzyme known as telomerase. Experiments on the functions of telomerase may lead to new understanding of the aging process, the replication ability of cancer cells, and mortality.