BRCA1 and cancer: However, this conclusion is not entirely consistent with the conclusions of Xu et al. They proposed that the increased sensitivity of cancer cells with defects in the HR pathway (e.g., loss of BRCA1/BRCA2) to CX-5461 results from CX-5461’s stabilization of G4 structures in telomeres, causing replication fork blocking and the induction of ssDNA gaps and breaks.