Lesions formed in a close proximity (i.e., within a few nm) result in clustered types of DNA damage, also called multiply damaged sites (MDS) and are considered the fingerprint of IR. Clustered DNA lesions can comprise a DSB and several base damages and/or abasic sites in close vicinity. In the case of multiple DSBs, we refer to the idea of complex DSBs . The biological significance of such lesions relates to the inability of cells to process them efficiently compared to isolated DNA damages and the outcome in case of erroneous repair can vary from mutations up to chromosomal instability . Therefore one should wonder if there are any mutational signatures of IR. Only recent evidence, mostly due to availability and affordability of next generation sequencing technologies, indicates that such radiation signatures do exist ; yet previous studies have shown the lack of such associations . Specifically, Behjati et al. have shown a significant increase in small chromosome deletions and balanced inversions in radiation-associated tumors which probably act as driver mutations and explain the carcinogenic potential of IR. More importantly, they suggest that these chromosomal abnormalities originate from the repair of radiation-induced DNA damage via the less accurate pathways of non-homologous (NHEJ) or microhomology mediated end-joining (MMEJ) . Therefore, accepting the claim that IR induces complex DNA damage that is irreparable and leads to mutations or structural abnormalities and subsequently to genomic instability (GI) and cancer, radiation-induced cancers should bear traces of the radiation-related origin of these mutations. Disruption of genome maintenance (i.e., GI) can occur through a variety of mechanisms and it is now considered as a key hallmark of cancer. Hence, there is a great need for improved detection techniques at cellular and tissue level that will provide valuable information for understanding the cellular mechanisms to process clustered DNA lesions .