Functional characterization of TERT rare variants. (A) Cell-based telomere elongation assay in isogenic bulk K562 cell lines with doxycycline-inducible TERT expression. TERT expression throughout the experiment is shown for control conditions: luciferase, TERT^WT, TERT^V694M. hTERT band (∼127 kDa) is labeled with an arrow, and the asterisk corresponds to a nonspecific band seen in all conditions. (B) Telomere length measurements by terminal restriction fragment analysis and qPCR (ddCT represents RTL) for luciferase (green), TERT^WT (blue), and TERT^V694M (red). (C) Telomere elongation capacity of TERT rare variants normalized to TERT^WT are shown in ranked order grouped by structural domain. Control conditions are colored in black and TERT rare variants in red.