GPT and cancer: As expected telomerase positive cancer cells (i.e. HeLa) produced a robust signal in the ddTRAP reaction compared to the absence of signal in telomerase negative cell lines such as BJ and IMR90 fibroblasts, human papillominavirus (E6/E7) transformed BJ fibroblasts and alternative lengthening of telomeres (ALT) cancer cells (Figure 5; (23); qualitative gel-images of most cell lines used are shown in Supplementary Figure S4).