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Thermodynamic instability is a problem when assembling and purifying complex DNA nanostructures formed by hybridization alone. To address this issue, we have used photochemical fixation and orthogonal copper-free, ring-strain-promoted, click chemistry for the synthesis of dimeric, trimeric, and oligomeric modular DNA scaffolds from cyclic, double-stranded, 80-mer DNA nanoconstructs. This particular combination of orthogonal click reactions was more effective for nanoassembly than others explored. The complex nanostructures are stable to heat and denaturation agents and can therefore be purified and characterized. They are addressable in a sequence-specific manner by triplex formation, and they can be reversibly and selectively deconstructed. Nanostructures utilizing this orthogonal, chemical fixation methodology can be used as building blocks for nanomachines and functional DNA nanoarchitectures.

International audience; Soft tissue sarcoma (STS) is one of the most frequent second primary cancer that occurs during the first 20 years following treatment for a solid cancer in childhood. Our aim was to quantify the risk of STS as a second malignant neoplasm and to investigate its relationship with radiotherapy and chemotherapy. A cohort study of 4,400 3-year survivors of a first solid cancer diagnosed during childhood in France or the United Kingdom, between 1942 and 1985, was followed 15 years on average. In a partially nested case-control study, we matched 25 cases of STS and 121 controls for sex, type of first cancer, age at first cancer and duration of follow-up. Sixteen STS occurred in the cohort, as compared to 0.3 expected from the general population (Standardized Incidence Radio, SIR = 54 (95%CI: 34-89)). The SIR was 113 (95% CI: 62-185) after chemotherapy plus radiotherapy (13 STS), whereas it was 28 (95%CI: 2-125) after chemotherapy alone (1 STS) and 19 (95%CI: 3-60) after radiotherapy alone (2 STS). After adjustment for treatment, there was no evidence of variation in the annual excess of incidence or in the SIR with either age at first cancer or time since 1st cancer. In the case-control study, the risk of a STS was increased with the square of the dose of radiation to the site of STS development and with the administration of Procarbazine. The increased risk of soft tissue sarcoma that occurred after childhood cancer is independently related to exposure to radiotherapy and Procarbazine. A closer surveillance of children treated with this treatment combination is strongly recommended.