Telomere fusion occurs between very short telomeres and is characterized by extensive deletion and microhomology at the fusion point. (A) DNA sequences of 2 telomere fusion events involving the XpYp and 21q telomeres obtained from CLL B cells. Microhomology at the fusion point is underlined and bold. The amounts of TTAGGG repeats and variants are denoted as T (black) and V (red), respectively. Deletion size (bp) is indicated. (B) DNA sequence of a clonal fusion event. Two identical sequences were from separate fusion molecules obtained from the same patient. (C) Histogram summarizing the amount of contiguous TTAGGG repeats adjacent to the fusion point. (D) Summarizing the amount of deletion into the telomere-adjacent DNA determined from the start of the telomere repeat array. (E) Summarizing the amount of 100% homology between the fusion partners at the fusion junction. Additional homology is observed adjacent to the fusion point (supplemental Figure 3).