The enzyme telomerase maintains telomere length by adding TTAGGG repeats to the ends of chromosomes when DNA replicates.

Although highly expressed in stem cells, telomerase is hardly detectable in differentiated cells. It is reactivated during tumorigenesis, however, and appears to be associated with cell immortalization. Kanaga Sabapathy and co-workers now provide evidence that it also confers resistance to apoptosis (see p. 819). They have ectopically expressed the catalytic subunit of telomerase (TERT) in mouse embryonic stem (ES) cells. Significantly, this does not affect the ability of the cells to differentiate (which some previous results had suggested). But it does protect them against apoptosis that occurs during differentiation. Furthermore, the authors show that TERT confers resistance to apoptosis induced by oxidative stress and chemotherapeutic agents. They go on to demonstrate that it offers little protection against cell death in ES cells that lack the tumour suppressor p53, which is normally upregulated when these cells undergo apoptosis. Sabapathy and co-workers therefore conclude that telomerase provides resistance to p53-dependent apoptosis in ES cells. They also propose that it might thus represent a useful basis for improving the viability of stem cells for transplantation.