For example, in a considerable proportion of CLL clones (~ 20%), the ATM gene is mutated; the ATM gene product is known to be involved in the repair of DNA double-strand breaks (Dunst et al. 1998; Humphreys et al. 1989; Jones et al. 1995; Parshad et al. 1985; Stilgenbauer et al. 2000), and mutations of this gene are associated with increased vulnerability to the carcinogenic effects of ionizing radiation. Here, ATM is linked to B-cell chronic lymphocytic leukemia.