Lessons from Mice without Telomerase

T he observation that almost all malignant cancers have telomerase activity has been explained by the assumption that telomerase is crucial for the progression of malignancy. Surprisingly, cells from mice without a functional telomerase RNA gene and no telomerase activity are indistinguishable from normal cells in different transformation and immortalization assays. However, detailed analysis of chromosomes in telomerase null cells has revealed multiple defects that point to the role of telomerase in normal biology and raise well-defined questions for future research.