Direct telomerase assays were conducted with 5 nM 32P-end labeled primer a and d Primer 3 (AGGGTT)3, b Primer 1 (TTAGGG)3, or c Primer 2 (AGGGTT)3. Reactions contained 50 μM dATP, dTTP, and dGTP, except when 2-OH-dATP (2dA) or 6-thio-dGTP (6dG) was substituted for dATP or dGTP, respectively, where indicated. Products were separated on denaturing gels. Letters on the y-axis of the graph indicate the template base; P indicates unextended 18-mer primer. Graphs represent percent of product terminated at each template position as a function of total products. Images are representative of, and data are the mean ± s.d. from, three independent experiments. Source data are provided as a Source Date file.