In this study, we compared AT clonal cells, representing 88% of the entire T lymphocytes (AT94-1) and expressing TCL1 oncogene (ATM− TCL1+), cell cycle progression to T lymphocytes of AT patients without TCL1 expression (ATM− TCL1−) by analysing their spontaneous apoptosis rate, spontaneous telomerase activity and telomere instability. The gene discussed is TCL1B; the disease is ataxia telangiectasia.