However, the sensitivity to the Chk1 inhibitors was not just limited to the TNBC cell lines as the HER2-postive, ER-negative SKBr3 breast cancer cell line and the SKOV-3 ovarian cancer cell line (HER2-positve, ER-positive but estrogen insensitive [40]) were among the most sensitive cell lines to Chk1 inhibitor induced cell death. This evidence concerns the gene ESR1 and breast cancer.