CLL patients displaying short TTAGGG repeat lengths exhibit telomere fusion. (A) Scatter plot displaying XpYp telomere length in terms of TTAGGG repeats distal to the TVR region. Persons marked in color were analyzed for fusion: red represents fusion was detected; and blue, fusion was not detected. (B-C) Telomere fusion assay in one normal person and one stage A patient, both of whom show no evidence of telomere fusion. (D-E) Examples of telomere fusion in stage A and stage C patients. Each band represents a single fusion event. Fusion was detected by Southern hybridization with the XpYp telomere-adjacent DNA probe. Red arrows indicate an example of a clonal fusion event, verified by sequence analysis (Figure 3C). (F) Fusion assay applied to MRC5 fibroblast cells expressing HPVE6E7 entering crisis in culture⁶ ; PD points are indicated above. Frequencies of fusion of the XpYp telomere with a subset of 13 chromosome ends are indicated below each blot. These frequencies are useful as a comparator of relative fusion frequencies but represent considerable underestimate of the genome-wide frequency of telomere fusion.