Telomere and telomerase status in normal and malignant hematopoietic cells. LSCs are distinct from LT-HSCs and ST-HSCs. LT-HSCs have relatively long telomeres, but low telomerase activity, which cannot maintain telomere length, and thus LT-HSC telomeres shorten when the cells replicate. ST-HSCs have long telomeres and up-regulate telomerase, enabling them to actively amplify. LSCs have high telomerase activity but short telomeres that essentially require maintenance by telomerase, thereby providing a target for therapeutic intervention.