Comparisons of telomere lengths in B cells and PMNLs using Flow-FISH. (A) Region R1 identifies the B-cell and PMNL populations under study based on forward scatter. (B) Dotted open histogram represents background fluorescence of cells subjected to Flow-FISH in the absence of the telomere-specific PNA probe; the shaded histogram represents telomere-specific fluorescence of cells. The difference between the mean fluorescence intensity of cells stained with and without probe (as deduced from marker M1) yields mean telomere fluorescence and is used to calculate telomere length in kilobases (see “Patients, materials, and methods”). (C) Mean telomere lengths of paired PMNLs and B-cell populations in 15 B-CLL patients and 15 age-matched healthy donors. The differences in telomere lengths between PMNLs and B-cell populations from B-CLL patients were statistically significant (P < .01), whereas telomere lengths did not differ in B cells and PMNL populations from healthy subjects.