ATR and Ewing sarcoma: Whole exome sequencing of Ewing sarcoma cell lines revealed an absence of mutations in DNA repair genes, and as both ATM and ATR DDR signaling pathways remain functional in Ewing sarcoma, PARP inhibitor sensitivity is not underpinned by mutational defects in DNA repair by homologous recombination but perhaps through hypersensitivity to trapped PARP-1 DNA complexes61.