Callen et al. (2002) studied several markers of telomere integrity and function in lymphocytes of FA group A patients and age-matched controls. A higher frequency of extrachromosomal TTAGGG signals and of chromosome ends with undetectable TTAGGG repeats were observed in FA cells by FISH, suggesting intensive breakage at telomeric sequences. Consistent with previous reports, quantitative FISH analysis showed an accelerated telomere shortening of kb in both arms of FA chromosomes. A 10-fold increase in chromosome end fusions was observed in FA cells, despite normal binding of TRF2 ( 602027 ), a telomere binding factor that protects human telomeres from end fusions. The authors concluded that telomere erosion in FA is caused by a higher rate of breakage at TTAGGG sequences in vivo in differentiated cells, and that the increased occurrence of end fusions is independent of TRF2 binding.