LETTERS TO THE EDITOR addition, whereas we did observe a shift in telomere lengths to shorter sizes during serial transplantation of HSCs from nontransgenic mice, we did not observe an increase in ends lacking detectable telomere signal (Fig. 1e). Furthermore, we did not observe any end-to-end chromosomal fusions in cultured transgenic or nontransgenic HSCs from long-term reconstituted fourth generation recipients, although we (RCA and ILW, unpublished observations) and others8 have observed signs of telomere dysfunction, including end-to-end chromosomal fusions and chromosome ends lacking detectable telomere signal, after transplantation of HSCs from telomerase-deficient mice.