In this review, we summarise the function and structure of telomerase reverse transcriptase (
TERT) in humans, including its regulation. The dysregulation of telomerase through TERTpromoter mutations across a range of cancers is discussed. The molecular mechanism activated by TERTpromoter mutations is outlined. Finally, the timing of TERTpromoter mutations during carcinogenesis is reviewed in the context of their potential utility as clinical biomarkers of malignant transformation.