With regard to the moderate but significant radiosensitivity observed with the AT-like disorder (ATLD) (MRE11 mutations), Fanconi anemia (FANC genes mutations), Bloom’s syndrome (BLM mutations), and xeroderma pigmentosum (XP genes mutations), it must be stressed that all these mutations affect the repair of DNA mismatch (DM), base damage (BD) or DNA single-strand breaks (SSB) that may be involved in the cell death or interplay with DSB repair pathways. The gene discussed is MRE11; the disease is xeroderma pigmentosum.