ATM and B-cell chronic lymphocytic leukemia: In CLL, the majority of TP53 defects consists of biallelic TP53 defects (70%), that is, a TP53 deletion (17p deletion) in one allele in conjunction with a TP53 mutation in the other allele.5 In marked contrast, less than 40% of ATM deletions (11q deletion) coincide with an ATM mutation.2 Whereas monoallelic lesions of TP53 (i.e., mutations or deletions) commonly lead to p53 dysfunction and impaired responses to chemotherapy,5, 10 only biallelic defects of ATM (i.e., mutation and deletion) usually result in impaired p53 response.2, 4, 11