Frequency of ultrashort telomeres in subpopulations of hematopoietic cells. Bar graphs show the frequency of statistical outliers within each sorted subpopulation. Where no bar is shown, no telomere data exist for that particular subpopulation. The frequency of statistical outliers in more differentiated cells was compared with the most primitive cells available for each sample. Linear regression analyses were performed to identify significant increases in ultrashort telomeres, with significant subpopulations denoted as ■. MPB subpopulations showed significant increases in ultrashort telomeres with respect to more primitive cells; however, this trend was not observed in CB samples. An additional 2 CBs were analyzed and also showed no significant increases in ultrashort telomeres (data not shown).