Another factor that can contribute to the progression of cancer is the inappropriate activation of the enzyme telomerase. Recall from Chapter 9 that in most somatic cells, the ends of chromosomes cannot be replicated, and the telomeres become shorter with each cell division. This shortening eventually leads to the destruction of the chromosomes and cell death, so somatic cells are capable of only a limited number of cell divisions.

In germ cells and stem cells, telomerase replicates the chromosome ends (see Chapter 9), thereby maintaining the telomeres, but this enzyme is not normally expressed in somatic cells. In many tumor cells, however, sequences that regulate the expression of the telomerase gene are mutated, allowing the enzyme to be expressed and the cell to divide indefinitely. Although the expression of telomerase appears to contribute to the development of many cancers, its precise role in tumor progression is unknown and is under investigation.