Disruption of TERT in donor HSPCs demonstrates telomere shortening and proliferative disadvantage in vitro and in vivo. (A) A schematic of the in vitro and in vivo assays to study the effect of telomere shortening caused by monoallelic or biallelic TERT disruption in donor HSPCs. (B) Telomere length analysis of CFU colonies derived from donor HSPCs 3 days after nucleofection. Mann-Whitney test. NS: P > .05 and **P < .01. (C) Percentages of frameshift alleles of bulk hCD34⁺ cells before and after xenotransplantation. Each replicate shows data from 1 biological replicate.