TPP1 and Infertility: Infertility has been reported in mTR−/− mice (Lee et al, 1998), but reproducing this phenotype with a mutation in TPP1 that solely disrupts recruitment of, but not the expression or composition of, telomerase confirms beyond reasonable doubt that telomerase-dependent telomere shortening is sufficient to cause mouse infertility in the absence of concomitant spontaneous HSC failure.