TPP1 with ΔK170 mutation does not recruit telomerase to telomeres. (A) FISH using probes against telomeres and hTR in 293T cells treated with siRNA against endogenous TPP1 and expressing either WT or mutant TPP1 or a vector-only control. (B) Quantitation of the number of hTR/telomere colocalizations in 3 independent transfections and FISH experiments (100 cells [300-1000 colocalizations] counted for each sample in each experiment; data shown as mean ± standard error of the mean; control had 7.8 ± 1.2 colocalizations; *P = .024).