Overexpression of TERT and TR in tagged hESCs suggests potential postassembly defects for N-terminally tagged telomerase. (A) Targeting schematic of GFP, TR, wt TERT, or Flag-TERT overexpression from the AAVS1 locus in endogenously targeted hESCs. “g500” in the TR overexpression plasmid indicates 500 bp downstream of the genomic hTR locus. (B) Telomere restriction fragment assay of targeted wt, Flag, or YbbR hESCs overexpressing GFP, TR, wt TERT, or Flag-TERT at the AAVS1 locus. Samples were collected 3, 4, and 5 weeks (passages) after the targeting.