Upregulation of the telomerase reverse transcriptase () gene in human cancers leads to telomerase activation, which contributes to the growth advantage and survival of tumor cells. Molecular mechanisms of upregulation are complex, tumor-specific and can be clinically relevant. To investigate these mechanisms in breast cancer, we sequenced the promoter, evaluated copy number changes and assessed the expression of the oncogene, a known transcriptional regulator, in two breast cancer cohorts comprising a total of 122 patients. No activating promoter mutations were found, suggesting that this mutational mechanism is not likely to be involved in upregulation in breast cancer. The T349C promoter polymorphism found in up to 50% of cases was not correlated with expression, but T349C carriers had significantly shorter disease-free survival. gains (15-25% of cases) were strongly correlated with increased mRNA expression and worse patient prognosis in terms of disease-free and overall survival. Particularly aggressive breast cancers were characterized by an association of gains with overexpression. These results evidence a significant effect of gene copy number gain on the level of expression and provide a new insight into the clinical significance of and upregulation in breast cancer.