Telomerase - an enzyme that endows cells with unlimited proliferative potential - is differentially expressed in cancer cells and in normal cells. Although most primary human cells lack telomerase, the enzyme is upregulated in more than 90% of invasive breast cancers. As a result, much of breast cancer development occurs before telomerase is reactivated during a critical transition from a telomerase-negative to a telomerase-positive state. During this transition, the telomere shortening that accompanies cell division may either prevent or facilitate tumorigenesis by activating checkpoints and impairing chromosomal stability. In mature cancers, telomerase probably serves a crucial role in tumor progression and maintenance by stabilizing telomeres and supporting the immortal growth of breast cancer cells.