Telomere length of human satellite cells. Telomere length was measured by flow cytometry using fluorescence in situ hybridization without (A) or with the fluorescein-conjugated PNA probe (B). Propidium iodide (PI) was used for labeling DNA content. The telomere probe signal was measured in the FL-1 channel and the PI signal in the FL-3 channel. G0/1 phase sample and control cells were gated according to their PI specific signal on a dot blot (FL1-H vs. FL3-H). Relative telomere length (RTL) was calculated by: RTL = (mean FL1 sample cells with probe - mean FL1 sample cells without probe)/(mean FL1 control cells with probe - mean FL1 control cells without probe) * 2 * 100%. Representative images are shown of the one of the RA patients. For each sample analyzed 20,000 counts were acquired.