The complexity of DNA damage as discussed above refers to the idea of clustering of several and different DNA lesions within a short DNA region of 10–15 bp. The two main categories of lesions appearing in a cluster are the DSB and non-DSB lesions, usually referred to as oxidatively-clustered DNA lesions (OCDLs). The reader can refer to several comprehensive reviews for the general description of clustered DNA damage , detection methodologies  and biological importance. More specifically for the accepted repair resistance of these lesions, including experimental evidence on the increase of the possibility for generation of mutations and chromosomal breaks after erroneous repair of clustered DNA lesions please see . Although within the context of this review we refer to bistranded DNA lesions appearing in both the DNA strands, there is also the possibility of unistranded or tandem lesions appearing in the same DNA strand and several groups have dealt with the processing and biological role of these complex DNA lesions as described in recent reviews .