Table 2.

Atm deficiency results in chromosomal instability

Genotype	Breaks per metaphase (± SE)	n
+/+	0.71  ±  0.22	28
+/−	0.88  ±  0.28	17
−/−	4.56  ±  0.48 (× 6.4)	52
+/+, Bleomycin	1.70  ±  0.39	20
−/−, Bleomycin	9.71  ±  0.99 (× 5.7)	41

Metaphase chromosomes were prepared from embryonic fibroblasts of wild-type (+/+), heterozygous (+/−), or homozygous (−/−) Atm mutant mice, either with or without bleomycin treatment. Data are presented as the average of the number of double-stranded breaks observed per metaphase. The fold-increase in Atm homozygotes vs. wild-type controls is also noted.