Mean telomere lengths in subpopulations of MPB and CB. Representative MPB and CB samples are shown with mean telomere lengths (± SEM) in all analyzed subpopulations. Primitive subpopulations show the longest mean telomere lengths within each sample. Of the terminally differentiated cell types, CD56⁺ cells routinely have the shortest telomeres. CD19/20⁺ cells have the longest mean telomere length of terminally differentiated cells, predominantly due to a subset of cells having very long telomeres. This has been observed elsewhere and is presumably due to the reactivation of telomerase during B-cell development. Full telomere length data and distribution from all samples analyzed are presented in Figure S1 (available on the Blood website; see the Supplemental Materials link at the top of the online article).