The enzyme telomerase catalyzes the de novo synthesis of telomere repeats, thereby maintaining telomere length, which is necessary for unlimited cellular proliferation. Telomerase reverse transcriptase (TERT), the catalytic domain of telomerase, is the rate-limiting factor for telomerase activity and is expressed in virtually all tumors. Thus, TERT has been proposed as a marker with diagnostic and prognostic potential in breast cancer as well as a basis for breast cancer therapeutics. In these contexts, it is important to define the sites and extent of TERT expression in normal and cancerous human breast tissues. In this study, levels of TERT mRNA were measured within a set of 36 breast carcinomas and 5 normal breast samples by quantitative real-time reverse transcription-PCR, and we subsequently identified and characterized the cells expressing TERT mRNA within these tissues using in situ hybridization. The results show that (a) detectable TERT mRNA expression is specific to the epithelial cells; (b) TERT is expressed in both normal and malignant breast tissues; (c) the pattern and level of TERT expression are heterogeneous, with approximately 75% of tumors expressing bulk TERT mRNA levels equal to or less than those within normal breast tissue; and (d) tumors expressing above-normal levels of TERT mRNA are more likely to be histopathologic grade 3 (P = 0.002), contain high fraction of cells in S phase (P = 0.004), and have increased levels of MYC mRNA (P = 0.034).