**6. Molecular nature of radiation signature involved in radiation-induced genomic instability**

Radiation-induced genomic instability is transmitted through many generations after the initial X-irradiation, indicating that there should be some mechanism(s), by which the initial radiation insults are memorized (Suzuki, 1997). As delayed DNA double-strand breaks are identified in surviving cells, persistent DNA damage is one candidate for radiation signature. In fact, a recent study has proved that DNA double-strand breaks induced by very low dose of X-rays remain unrepaired for many days in confluent cells (Rothkamm and Lobrich, 2003). Furthermore, if persistent DNA is radiation signature, radiation-induced genomic instability is more prevailing in xrs-5 and xrs-6 cells compared with CHO cells. But, we observed radiationinduced genomic instability at similar level between these cells (Suzuki et al., 2009). Thus, these results exclude the possibility that radiation signature is DNA double-strand breaks *per se*. Previously, we compared delayed aberrations of X chromosome with or without large deletion at the hypoxanthine-quanine-phosphorybosyltransferase gene, which is located at Xq26.1 (Toyokuni et al., 2009, Suzuki et al., 2003). Because cells defective in the gene were able to grow in the presence of 6-thioguanine, we cloned 6-thioguanine resistant clones after 3 Gy of X-rays, and examine gene deletion. We found some of the resistant clones had large deletions expanding over several megabases. Interestingly, these clones with large deletions showed higher probability to induce delayed chromosomal instability. Thus, altered higherorder chromatin structure could be a candidate for radiation signature. It is possible that such altered higher-order chromatin structure results in replication stress, which causes DNA double-strand breaks.
