The induction of DNA interstrand cross-links by ionizing radiation has been 
largely ignored in favour of studies on double-strand break formation and 
repair. At least part of the problem is technical; it is difficult to detect and 
quantify interstrand cross-links when the same agent forms both cross-links and 
single strand breaks because the detection of interstrand cross-links generally 
involves a denaturation step. Our group has studied the induction of interstrand 
cross-links following irradiation of DNA containing bromouracil at specific 
sites. We found that the formation of interstrand cross-links requires the 
presence of a few (3-5) mismatched bases, comprising the bromouracil. In the 
absence of mismatched bases, no radiation-induced cross-linking was observed; 
however, even in the absence of bromouracil, cross-linking still occurred, 
albeit at a lower efficiency. Our molecular modelling studies demonstrate that 
the mobility of the bases in the mismatched region is essential for the 
cross-linking process. Thus, our hypothesis is that ionizing radiation induces 
DNA interstrand cross-links in non-hybridized regions of DNA. Some obvious 
examples of such DNA regions are replication forks, transcription bubbles and 
the D-loop of telomeres. However, an abundance of studies have made it clear 
that there must be many single-stranded regions in the genome, such as hairpins 
and cruciforms. For example, alpha satellite DNA, in centromere regions of human 
chromosomes, forms hairpins. Thus, a variety of non-B DNA structures (hairpins, 
slipped DNA and tetrahelical structures) exist in the genome and should be 
susceptible to the formation of radiation-induced interstrand cross-links. 
Although interstrand cross-links have thus far been virtually ignored in 
radiation biology, it will be worthwhile to develop methods to detect their 
presence following exposure of cells to biologically relevant levels of ionizing 
radiation, since, on a per lesions basis, they are probably more toxic than 
double-strand breaks.