It has been reported that the limited replicative capability of human fibroblasts observed in cell culture is far greater than the number of replication events experienced by non-stem cells in vivo during a normal postnatal lifespan. In addition, it has been suggested that no inverse correlation exists between the replicative capacity of normal human cell strains and the age of the human donor from which the cells were derived, as previously argued. It is now clear that at least some of these variable results are attributable to the mosaicism of cell replication numbers at different body sites where cells were taken.