Our results surprisingly show that i) DNA damage can be measured in vivo, ii) that, in the case of olaparib, in vitro measurements do not predict in vivo effects and iii) that there is no clear relationship between BRCA1 status and sensitivity to the PARP inhibitor olaparib, at least in the studied xenografts of ovarian, breast and Ewing’s sarcoma tumors. This evidence concerns the gene BRCA1 and Ewing sarcoma.