Owing to the important roles played by the upregulation of Cyclin D1 and the activation of HR in rendering BC cells insensitive to olaparib and considering our previous finding that the repression of BC cell proliferation by YB-004 is achieved by suppressing Cyclin D1, which inhibits HR, we further investigated whether YB-004 could also increase the sensitivity of BC cells to olaparib by inhibiting HR. This evidence concerns the gene CCND1 and breast cancer.